Matthew Norman: Let's hope Alan Johnson doesn't get asked about stamp duty

The mood music at which Johnners can excel is important. But you can't make music of any kind without knowing the individual notes

In the field of numbers more than any other, the spotlight can wreak havoc on the mind. Once a week some blushing nebbish does The Weakest Link walk of shame after telling Anne Robinson that 11 minus 2 equals 49, while I will never get over David Lammy, the then minister for higher education, enlivening Celebrity Mastermind with his assertion that Henry VIII was succeeded on the English throne by... Henry VII.

"It's so different when you're in the studio," goes the mantra of the humiliated contestant. "I shout out all the right answers watching at home, but I went to pieces under the studio lights."

So pity Alan Johnson, who appears to have the sort of grasp of economic facts and figures you might expect of Vicky Pollard, Phoebe from Friends, or the Tibetan mountain yak known to his mates as Innumerate Ian. In studios in recent days alone, the shadow Chancellor has advanced the novel theory that food is subject to VAT, and cited the rate of employer's national insurance contribution at 20 per cent... tantalisingly close to the correct 12.8 per cent, but no Monte Cristo No 3 all the same.

The poignancy here is that when watching Newsnight at home, all relaxed with a glass of whatever superannuated Mods drink to hand, he's an absolute whizz. He could teach Milton Friedman about money supply, and astound Keynes's ghost with his analysis of precisely how to calibrate public investment to avoid a recessionary slump. We know this because at Monday's news conference Ed Miliband told us so. "Alan," he insisted, "clearly knows about these things."

Now admittedly some regard the words "clear" and "clearly", as so often deployed by politicians of all parties, as a coded method of diametrically reversing the overt meaning of a sentence. It is a bit like the Radio Times formulation for Last of the Summer Wine which goes "Gentle comedy from the pen of Roy Clarke", where "gentle" is a polite synonym for "no". Similarly, the cynics and sneerers claim, whenever neo-clarity trail-blazer Hazel Blears said: "Now Jeremy, I have been absolutely clear about this...", she was subtly conveying the message: "Now Jeremy, I have blethered obtuse and incomprehensible gibberish about this because I haven't one clue what I'm doing."

Even if there is some truth in this, that was then. Now Labour is led by a peachy new generation which eschews the ways of the old one, so when Ed M says "clearly" he means it literally. Let me be abundantly clear about that because the alternative is hideous: if Ed used it in its Blearsian sense, he confessed to appointing to the most pivotal opposition post the laziest, most dilettante frontbencher in memory.

On the September day he did so, Johnners said he needed time with his economics primer, which was droll and charming in the geezer-dahn-the-pub way that makes the chilled-out postie such an effective antidote to the tediously over-earnest technocrats Ed Balls and Yvette Cooper whom Miliband overlooked in his favour. But if four months later he hasn't bothered to dip into that textbook, preferring to wing it Sarah Palin-fashion with a few notes hurriedly scrawled on his palm, this is something between a problem and a catastrophe for his leader.

Opposition at its core is about persuading an electorate of your competence. The first and paramount requirement is to implant in the national consciousness the nebulous idea that you know what you're doing, thereby enabling the punters to imagine you in power without snorting coffee through their nostrils in derision. Margaret Thatcher and Tony Blair did that. Neil Kinnock and William Hague did not.

Clearly, in its pre-Blears sense, it does Ed M savage harm if public opinion holds that he handed the Treasury portfolio, during such a vicious economic cycle, to a man who sits in front of the telly not shouting out the right answers; but bemusedly muttering, "Ah yes, capital gains tax, that rings a bell. Isn't that the one you pay at the airport coming home when you've put on weight on holiday in Oslo or Bangkok?"

"It's the big things that matter in politics," was how Little Ed developed his Johnners defence on Monday. "The things that matter are your instincts." How very true. It was their instincts that led George W Bush and Mr Blair to invade Iraq. Yet in economics as well as in warfare, the detail can have its role to play too.

The mood music at which Johnners can excel is important, but – as he of all frustrated guitar hero wannabes should know – you can't make music of any kind without knowing the individual notes. Not unless, like Eric Morecambe in the André Previn sketch, you're playing it only for laughs. With Johnners, the suspicion isn't so much that he doesn't necessarily play them in the right order, but that he hasn't mastered his economic do-re-mi.

Let me be absolutely clear that this suspicion is ill-founded, and that it is simply him freezing under the spotlight. So assuming Little Ed thinks it too degradingly soon to send him on the walk of shame, I suggest he looks to fictional quiz show history for a makeshift solution.

In an episode of The Phil Silvers Show, the masterful Sergeant Bilko's latest potential goldmine is a new motor platoon recruit with astounding knowledge of American birds. Bilko wangles him a spot on The $64,000 Question, but just before his crack at the grand prize he loses his nerve and memory. So he fits the fellow up with a two-way radio microphone in the form of an earpiece, and back at the barracks Doberman and the rest watch the live show with every bird encyclopedia ever published at hand, relaying the answers to the man in the soundproofed booth.

Ultimately it didn't work out for Bilko (nothing ever did), and the tactic would hardly hit the jackpot for Little Ed. But this is now purely about damage limitation, so the next time Alan Johnson appears in a studio to discuss matters economic, there must be someone at Labour HQ whispering into his earpiece: "Ok, Alan, try to relax, Paxo's asking you about stamp duty. Now that's a tax on property transactions, just like we discussed earlier. Please don't repeat what you said in shadow Cabinet about it being the moral obligation all postal workers feel to deliver the mail on time."