Sian Berry: Will the woman described as 'environmental Viagra' turn us on to the Green Party?

Where are the Greens when you need them? The climate-change clock is ticking, and globally, we're told, it's 10 to midnight. Locally, outside London's grand Methodist Central Hall, it's definitely 7.12pm, but there's still no sign of Sian Berry. The London Citizens' mayoral hustings is starting in three minutes and the audience has piled through the doors, bombarded on their way by political flyers of every colour... except Green. Where has she got to? Is she riding here by milk float?

Here she comes round the corner in her brown Terra Plana eco-boots (tanned without chrome, don't you know). She has a jaunty, studenty gait, all hip and nimble. Her clothes are shades of peat bog, lichen and country mulch, but her hair is pure gold from the roots. Boris Johnson is not the only natural blonde in this election. She looks lit from within, extraordinarily happy for someone who has, to date, lost three council elections and one parliamentary seat, and is currently spending every waking hour fighting an election that no one thinks she can win.

But belief sustains, and Berry believes in her policies, even the one about closing City Airport and turning it into an eco-housing village. Which would be "a huge blow to London's businesses", according to Boris. She thinks her party is steadily becoming more electable: "If this country had proportional representation, we'd definitely get Greens into the cabinet. We'd go all the way to the top."

But the Green Party has had many false dawns. Berry herself is reminiscent of Sara Parkin, a prominent figure in the party in the 1980s (along with David Icke, before the lizards got him). Could they really be about to turn the corner this time? "Well, we recently voted for a new party constitution," says Berry, "which means we will be electing a leader for the first time." This will help the party's profile enormously, I suggest. "The press will like it, won't they?" she concedes. "They love a good leadership contest."

She thinks a sea change is due at the next parliamentary election. "There are three constituencies where we should get in. The last election was too early." The party has since acquired some big names in Peter Tatchell (candidate for Oxford East) and MEP Caroline Lucas (candidate for Brighton Pavilion). And they have Sian. Lovely Sian, once described as "environmental Viagra". Committed Sian, who set up the Alliance Against Urban 4x4s, publicising the cause with spoof parking tickets and fancy-dress protests. Sensible Sian, who went to Pate's grammar school in Cheltenham (where both her parents taught) and has an engineering degree from Oxford. Clean-slate Sian, who claims to have had no political affiliation before joining the Green Party six years ago, aged 27. Is she too good to be true?

Her entourage at the hustings isn't impressive. She arrives trailing two helpers, intense young men in suits, and one hindrance – a bewildered-looking eco-blogger. "She's just been filming London Tonight," says one of the aides, explaining Berry's lateness. In fact, all the candidates arrive breathless. (This is the night Ken gets snapped straightening Boris's collar, an image passed around the country like a silly postcard in a classroom. You can't say this election hasn't added to the gaiety of the nation.) The aides are busy working out where they think Berry should sit in the huge ' tumult of the Methodist hall, when they realise she has already sat down. She's quick like that. Practical. Not the kind of politician whose secretary is their personal sat-nav. Not a man, in other words.

Up in the gallery, Justice For Cleaners are warming their throats for anti-Boris arpeggios. Tonight's hustings is for the four main contenders (and one of them a Green! This is progress indeed. Last London mayoral election, the Greens came seventh). All the candidates promise that, if elected, they will honour the assembled citizens' demands: for safe-streets initiatives, for a London minimum living hourly wage of £7.20, for more affordable housing and an illegal immigrant amnesty. Boris and Ken rouse passionate noises from the audience, some complimentary, some less so. Berry gets politely encouraging claps and whoops.

"Promising to stop dawn raids on children went down very well," one of those two intense young aides tells Berry, afterwards in the pub. (Wonks talk like this. It's a good sign that Green wonks are talking like this, too.) "Yeah, I enjoyed tonight," she says, sipping a pint of bitter and adeptly rolling herself a cigarette. "Apart from when they brought us bottled water on stage..." (Mayday, Mayday! Anti-mineral water campaigner in own goal alert!) "Generally it felt like home turf for me." Even though none of the issues was overtly environmental? "The Green Party has always been about social justice as well as the environment," she explains. I can feel the intense young aide sitting next to me clenching with frustration that this needs explaining. He's Scottish, Green and Socialist; Sian is a (non-active) member of the anti-capitalist party grouping, Green Left. She continues to explain, patiently,"There's an environmental aspect to housing, because to talk about affordable housing, you have to talk about insulation, which I would get free for Londoners – but I admit, there's nothing environmental about the London living wage. It's just about what's fair and right."

Berry has had a long day chasing those ideals. This morning she was in Mayfair's Berkeley Square at 7.30am, wearing a pith helmet and beating a drum as part of a stunt demonstration against the Porsche dealership. Breakfast, when she manages it, is Co-op "Shreddies" ("not actual Shreddies, cos they're from Nestlé"). She left her job running the Imperial College website in January and is living off campaign expenses. Home is a rented flat on a noisy bus route in Camden with her cat Sidney and her boyfriend, Jim Killock, a fellow Green ("I don't meet anyone else, these days"). He's the head of the party's communications; all the strategy runs by him. Handy. But not so good domestically: "I don't know who does the housework. No, I do: no one."

How did someone so normal end up in the Boris and Ken show? "I spent a long time shouting at Newsnight. By 1999 I'd become disillusioned with Labour, so I did some geeky research into other parties. The first Green meeting I went along to, I offered to stand. I haven't sat down since. I'm a volunteer. Always putting my hand up. And if people ask me to do something, I say yes. That's often the case with women, isn't it? Someone needs to encourage you at the start. I see women running single-issue campaigns, and I say to them, why don't you think of standing for council? Then you could make some decisions. Get some power."

As we leave the pub, Berry is detained by admiring wonks, hacks and policy geeks. "Sian, did you know a hotel has named a cocktail after you?" says one. "It's made of melon."

The next time I'm looking for Berry I follow my nose. At the Pepys Community Recycling Plant in Lewisham, she is inspecting the "wonderful" facilities that turn household kitchen waste into compost. But when I find her there among the shovels and steaming brown stuff, she's preoccupied. A bit agitato. "Have you seen Metro today?" She waves a copy. There's a story announcing that she plans to decriminalise cannabis, ecstasy and psychotropic mushrooms, and "provide heroin on prescription". Oh, and she would also legalise brothels. There's a picture of her wearing a red top, chosen, perhaps, because her eyes look fractionally sleepy. The message is clear: same old bonkers Greens. "Typical! This is just lazy journalism," she says, laughing but clearly riled. "Yes, these issues are on the Green manifesto. Yes, I support them. I believe in making life safer for prostitutes. But I'm not fighting this election on these points. My priorities are car clubs, more money for cycle lanes and 20p off public transport! These are the things I wanted to make clear at my press conference."

So why didn't she? Berry is extremely media-savvy. She recently persuaded Citibank to raise its cleaners' wages simply by threatening to hold a press call (she was going to deliver a pumpkin to their door on Halloween). She once dressed as a WAG to tell the 4x4 drivers, "Your Range Rover is so over". She knows how to work headlines. What went wrong here?

"I wanted to get up and talk about the issues I really care about, that are setting the agenda. But," she says exhaustedly, "I had to present the whole manifesto. I had to do it traditionally." Who made her do it that way – other Greens? "A group of about six or seven of us voted on it. We're democratic. We do things by consensus. I can't stamp my foot about it. I should have persuaded them through reasoned debate."

She wouldn't be the first to be frustrated by the party's idealistic inner workings. In the forthcoming internal elections, Berry can't stand for deputy as she is supporting Caroline Lucas's leadership bid, and the party has strict gender-equality quotas. This is fair and right, but really, is it politics? Machiavelli would be laughing in the aisles. "If Sian could meet every Londoner, most would warm to her. She would win the Mayoral election," Peter Tatchell told me. If, and if again.

Berry's more realistic aim on Thursday is to be elected a member of the London Assembly; then she could try to save this composting plant. A photographer from a local paper asks her to pose holding a compost bucket aloft. She gamely scoops up a handful of what was formerly kitchen waste, posing like a natural. "Just remember not to shake her hand," the photographer says to me, sotto voce.

Lunch on the hoof at a Vietnamese caff is fraught: they don't do veggie food or serve tap water. Over spring rolls, I quiz Berry on some of the more intriguing points in her manifesto, such as the free animal health service for London, and the animal blood banks. "Animal blood banks?" says another Green councillors now present. "Is that one of our policies? Do animals have different blood types, then?"

"Well," says Berry, "there's, I dunno, dog blood, and cat blood..." "But how far do you go?" asks the councillor. "Chipmunk blood? Guinea pig blood?"

"Budgie blood?" I pipe up. "The idea is you have a bit of everything in store..." sighs Berry, concluding proudly: "No other party offers animal blood banks." Then, whoosh, she's back to earth: "The idea is we would be a catalyst for these schemes and help secure their funding. The GLA [Greater London Authority] wouldn't be running them. But animal issues bring a lot of people to the party. So far Darren Johnson and Jenny Jones, the Greens on the London Assembly, have saved the London's Wildlife Crime Unit three times."

What, I wonder, does that do? Stop squirrels pilfering from bird feeders? "They address the smuggling of rare species into this country, often for bush meat or Chinese medicine." I stand corrected; that's a brilliant cause. But the cost must mount up. How would she pay for it all? "People were saying only four years ago, where are you going to get the money for insulation? Now it's come in from energy companies. It's there. Also, too much money is spent buttering up big business."

Berry's next stop is Peckham, where she will be handing out banknotes printed with her face on to emphasise not her majesty but the affordability of Green policies. "The price of oil is unstable, but renewable energy isn't," she says. "Green economies are resilient. Insulation saves you money. It all adds up."

And she's off to hit the streets. "I love flyering. I just look people in the eye, smile and they take my leaflet." Of course they do. There she goes in her eco-boots, to be worn till they fall apart like the last ones. What keeps her going, I wonder, through the media catcalls (The Times said with baffling cruelty that her make-up looked like a clown had applied it in the dark); the self-imposed eco-privations (she hasn't flown on holiday for 10 years); all the lost elections (she once missed out by a gutting 38 votes); all the exhausting trips to bottle banks? "Normally," she says, "it's the thought that no one else will do it. It might be up to me."

Eco worriers

The up-and-down progress of the Green Party

Once they were regarded as a bunch of wacky no-hopers; now they're a respectable alternative. The Green Party in Britain spent 14 years in the wilderness as a fringe grouping with no more influence than the Monster Raving Loonies of the late Screaming Lord Sutch, before bursting on to the national scene in one dramatic political event.

Formed in 1975 as the Ecology Party, they at first exemplified the old cartoon version of environmentalists as bearded, sandal-wearing lentil eaters (bottom left), although they did have one charismatic member in Jonathon Porritt (below right), a posh Etonian who had been teaching French in a comprehensive school and who became their first public figure.

They won 40,000 votes across Britain in the 1979 General Election, 55,000 in the election of 1983, and 90,000 in the election of 1987 (by now renamed the Greens) but were still a long way from prominence. Then came the shock (to the Greens as much as anyone else)...

In the elections to the European Parliament in 1989 they secured nearly 2,300,000 votes, or 15 per cent of the national total, pushing the Lib-Dems into a humiliating fourth place.

Britain had discovered environmentalism, the Green Party had arrived, and Porritt and his equally telegenic colleague Sara Parkin (pictured above right) found themselves national figures. But not for long: the "realists" in the party, including Porritt and Parkin, wanted a streamlined structure to fight elections and maybe win seats; the "fundamentalists" clung to the idea of the Greens as an "anti-party" party, and the two factions fell to squabbling.

After a disastrous performance in the General Election of 1992, the party fell into obscurity for most of the decade.

Their fortunes were revived in 1999 when two Greens, Caroline Lucas and Jean Lambert, were elected as British Euro MPs and the party at last had a taste of real influence beyond local councils.

They have made steady progress since in local elections. The next step: a seat at Westminster. The likeliest location is the constituency of Brighton Pavilion, where Councillor Keith Taylor won the Greens' highest ever vote in a national poll in 2005, with 22 per cent. Euro MP Caroline Lucas has been adopted to fight the seat next time. Watch this space.

Mike McCarthy, environment editor, The Independent