Not for the first time, but more now than ever, I commend to you V For Vendetta, the 2005 film in which a superhero in a Guy Fawkes mask sets about doing to Parliament what his role model tantalisingly failed to achieve under James I.

Slaughtered by most critics, the movie is hardly flawless in its totalitarian state allegorising, lacking the nuance of the Orwell novel that informs it, and its plot full of holes the size of Oliver Letwin's soggy tennis court.

For all that, I love it to bits, and suspect you might too, because in its crude and overblown way it caught a nebulous suspicion and gave it flesh. "The truth," V tells the watching public after hijacking the government-controlled state broadcaster, "is that there is something terribly wrong with this country, isn't there?... How did this happen? Who's to blame? Well, certainly there are those more responsible than others, and they will be held accountable. But, again, truth be told, if you're looking for the guilty, you need only look into a mirror."

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Those ultimately responsible for the democratic crisis of which expenses is the tiniest tip of the iceberg, in other words, are us. By that I mean not just us in the media, who until the Telegraph's glorious scoop largely abrogated our inquisitive duties. I mean you and me as private citizens. We have permitted the nonsense newly exposed in such detail, but known about and ignored for years, through sullen apathy. And we have allowed much else, and worse, besides.

One of the best things in the film is Stephen Fry, who gives a sparkling cameo as Gordon Dietrich, a TV satirist inspired to an act of suicidal defiance by V's broadcast. "He's right, isn't he?" says Dietrich after watching it. "There is something very wrong with this country."

Somehow, the memory makes Fry's intervention on exes this week painful to contemplate. In dismissing the furore as the carping envy of petit-bourgeois hypocrites, this clever, charming chap misses completely not only the genuine fury, but also the matter of how that fury must be channelled if a political system atrophied into incompetence, low-level corruption and the highest of farce is to be salvaged and rebuilt.

Only in moments of chaotic flux, when the foetid accommodations and stifling conventions of the age are suspended because the status quo looks scarier than radical change, does a glimpse of a less imperfect country feel like more than utopian dreaming. Such openings come seldom, vanish swiftly, and must be seized immediately.

This one may well not be. More than likely we will, until the June elections divert the spotlight, fixate on all the expenses debacle without questioning the underlying culture that generated it, and how that might be ended.

If so – if this golden opportunity is wasted – it will be a historic tragedy for this country. For the fiddling, as shameful as it's been, is not the disease but one of its more trivial symptoms. It is to a democratic sickness that remains largely undiagnosed what a bout of violent diarrhoea can be to colonic cancer. Mask it with over-the-counter medication though you may, more serious symptoms will soon enough emerge. The longer you ignore those, the more brutal the treatment required, and the lower the chances of recovery.

The illness in question is malignant in the extreme, and the only effective treatment is a written constitution. Since David Cameron will shortly be Prime Minister, it is to him we must turn on bended knee, begging that he acts while the rage is still hot and the desire for change intense, and makes a binding commitment to that constitution. He should pledge that, within an hour of kissing the Queen's hand, he will inaugurate a year-long national debate about how we want that constitution to look, involving the town-hall meetings and an appeal for public proposals with which we can reacquaint ourselves with the notion that our stake in how we are governed extends beyond voting with distaste every four or five years.

A committee of respected parliamentarians (there are a few) and distinguished outsiders – scientists, jurists, academics, trade unionists, soldiers, artists, and so on; even national treasures such as Mr Fry – should be co-opted to filter out the most promising ideas, and hand them to the Commons for a series of free votes. Those approved should then be given to a group of our finest writers, to be translated into a document as simple, elegant and enduring as the US Constitution revered almost as a deity to this day.

Several of the most compelling requirements race to mind. Electoral reform is one. The madness that the votes of little more a third of the actual electorate, and a fifth of the potential electorate, produce almost unlimited political power while disenfranchising the majority must end. Fixed-term parliaments are a no-brainer. So is a clause guaranteeing freedom of speech.

The soul-sapping spectacle of MPs trooping pliantly through the lobbies to vote for things in which they don't believe or actively disbelieve, or even of which they are blissfully ignorant, must stop. They continually assure us how incredibly hard they work, but being lobby fodder isn't work at all. It's a cushy version of house arrest.

The constitutional function of a backbench MP is not to rubber-stamp the leadership's will, but to act as a check against the power of the executive. A written constitution could enshrine their duty to vote according to conscience and constituents' interests, not the blackmail and bribery of the whips.

It should elevate the stature of select committees, those snivelling apologies for overseers of government practice and malpractice. If we drastically increased allowances for research staff, and offered additional salary, their members' status would be enhanced to approximately that of a minister of state. They might then resist the threats and lure of ministerial preferment, and do the fearlessly unpartisan job expected.

There are countless other symptoms that sorely want treating... the lack of an elected upper chamber; the absence of quasi-judicial scrutiny of such outrages as the decision to go to war in Iraq and the security failures that prefaced the 7/7 bombings; the refusal to devolve to local government outside London; the criminally reckless failure to control and de-politicise the police; and many more besides.

The overall imperative, however, is to treat the sickness itself by reconnecting the populace with its legislature, by restoring the supremacy of the Commons – our only direct link the central governance of Britain – by packing it with the kind of high-minded, talented and independent representatives whom we'd be delighted to pay £100,000 per annum and more.

These expenses-snafflers are not wicked people, but small people. I say this with no sense of superiority. I'm a small person myself. Most of us are. But these are big times beset by big issues, and we are sick to our eye teeth of leaving them to the dwarves.

We need that written constitution desperately. It is in David Cameron's gift, and his alone, meaningfully to promise one. He should do so at once. This window will close by the day, and may not reopen in our lifetimes.

m.norman@independent.co.uk