Carry on claiming: On Expenses

The MPs' expenses scandal had all the ingredients for a farce, yet a new television drama deftly balances the serious with the absurd

It was The Daily Telegraph what done it, right?

It was Middle England's favourite broadsheet that so uncharacteristically got its hands dirty and drip-fed us, morning after morning after morning, the sordid, petty, outrageous and often comic details of the MPs' expenses scandal. Well, yes, true in a sense – but where was all this information about the venality of Parliament coming from? Or, to put it another way, why was there any information available to be leaked in the first place? The short answer is this: Heather Brooke.

Brooke is the great unsung heroine of the Great Westminster Expenses Scandal, although that is about to change with the transmission of a BBC4 drama which details Brooke's five-year campaign to force MPs to come clean about their allowances. This is the other great thing about the story: it's generally unknown, says Tony Saint, the writer of On Expenses, which stars Anna Maxwell Martin as the plucky truth-seeker. It must have been quite frustrating for Brooke that no one seemed hugely bothered about it until the Telegraph started leaking the details. They got the totality of it and that became a story in itself – the scale became a story, I suppose.

Brooke had been an aspiring American journalist living in London when, in 2004, she started working on a book to coincide with the introduction of the Freedom of Information Act in the UK, a citizens' guide called Your Right to Know. Used from her student journalism days in the US to a transparent system of political expenses, Brooke took a keen interest in our own opaque parliamentary method of reimbursing itself.

"I've always been in love with old-style investigative journalism," she said last year, as the scandal claimed scalp after scalp. "You know those movies in the 1930s and 40s about the press in Chicago featuring hard-bitten hacks with hearts of gold, like Hildy Johnson in His Girl Friday? I always wanted to be that kind of reporter. I'm not sure if an onslaught of details about bath plugs and porn movies was how I envisioned the great investigation of my career, but then there are high principles at stake."

Indeed there are, and I must admit that my heart sank when I read that On Expenses was going to be a humorous drama about a story that has angered a lot more people than it has amused. What is it (the legacy of Private Eye and Spitting Image perhaps?) that means anything to do with Right Honourable Members (oo-er) must be automatically turned into a Carry On farce? "I was trying to capture what seemed to me to be a very British sense of humour about it all, says Tony Saint, who also wrote the jaunty 2007 Margaret Thatcher biopic, The Long Walk to Finchley.

Happily, On Expenses isn't any sort of Carry On Paying for My Duck Pond; it turns out to be pacy (the 60-minute running time helps) and entertaining primer to the whole sorry state of affairs, striking a good balance between drama and humour. But what really makes it stand out from your usual hot-off-the-headlines, kneejerk drama crowd is some serious casting.

Although they share only two wordless scenes together, Brian Cox as Speaker Michael Martin, the figurehead of parliamentary intransigence, and Maxwell Martin as his don't-take-no-for-an-answer nemesis complement one other beautifully.

Set up as the comic butt in early scenes – playing bagpipes in his office and mixing Irn Bru with his whisky – Cox endows Martin with an unexpected pathos. It will be up to those who actually know the man to say whether it is deserved, but Cox clearly believes he should be granted a fair hearing.

"Clearly he made errors in the way he handled the situation," Cox says. "But I also think there's never been any precedent for that kind of thing. And he was an old trade union man when a statesman was needed. But I think he was a kind of fall guy, and I wanted to play his humanitarian side."

Maxwell Martin, more used to portraying period characters in TV dramas such as Bleak House and North and South, is feisty, sexy and sporting an American accent quite unlike Brooke's. (You can check the real one out on YouTube.)

"Heather actually has a hybrid accent, half English and half American and I tried it make it more American because I didn't think her hybrid would translate very well," says the actress, adding that Brooke's nationality was the key to her determination. "She came over here and was shocked, really, at how much we bury our heads in the sand about things."

The obligatory disclaimer at the start of On Expenses is given an amusing slant, stating that this film is based on real characters and events. Some scenes have been imagined... some dates compressed. But mostly you couldn't make it up. The film's director, Simon Cellan Jones (Generation Kill; The Trial of Tony Blair) agrees. "It's quite a surreal story in that the people who voted in the Freedom of Information Act were the very people that tried to get themselves exempt from it."

On Expenses is on BBC4 on Tuesday, 10pm