How much gardening is necessary for Nick Clegg to carry out his job as leader of the Liberal Democrats? A lot? None at all? It is a question as absurd as the answer. For yesterday it was revealed that Sir Thomas Legg had provided a solution to this apparently impossible conundrum. The amount of gardening necessary for Mr Clegg to carry out his job is precisely £910 less than he claimed.

All this prompts a second question. How did a matter so ridiculous as the bill for an MP’s house cleaning come to dominate British politics?

It has happened for three reasons. The first is that somewhere between a handful and a couple of dozen MPs were engaged either in what can only be called “fiddling” of their expenses or in claims that showed they had totally lost touch with the lives of their constituents. A professional politician asking the taxpayer to fund a floating duck island has ceased to behave either professionally or politically. The number involved, while only a relatively small minority of MPs, was still shocking. This was certainly, therefore, a genuine scandal.

The second reason for the prominence of this scandal is the lamentable way that MPs have handled it. They have moved from an arbitrary set of rules enforced by the Commons Fees Office to a radically different but equally arbitrary set enforced by Sir Thomas. In both cases the thinking has been obscure, with no obvious reason for one claim to be granted and another rejected. And to this have been added infuriating attempts to shroud the whole thing in secrecy.

The final reason, however, is the biggest and it has nothing to do with who paid what for a soup spoon. Voters are angry with politicians. The Iraq war, the recession, real and growing income inequality, the payment of large amounts of tax for inadequate services, the intrusive nature of the modern State, a vague, intangible, feeling that somehow society is in moral decline: fury about all these things has found a voice and a cause with the allowances issue.

The public wonder whether MPs are worth paying at all. Their practices seem archaic and self-serving and their achievements underwhelming. Some of this is unfair, but, no matter how many reviews and inquiries are held, the allowances scandal will not go away fully until politicians understand this public mood.

So the furore over MPs’ expenses is real, understandable, and it will not disappear in a hurry. But yesterday we received further evidence that the fury has brought with it something as bad as the scandal that it is directed towards. There has been the rise of the idea that the country should be governed not by rules, but by the “court of public opinion” sitting in permanent session.

Because the application of the rules by the fees office proved unpopular, Sir Thomas has created a new set of rules and, extraordinarily, applied them retrospectively. Some of his judgments are baffling. Why should one MP be told that there is a new, backdated limit on cleaning bills, while another is informed that it is fine to charge up to the limit for his mortgage? Your house can be too clean, it appears, but not too large. Attached to these mysterious conclusions are massive bills — people on professional salaries are being asked to stump up thousands of pounds or be ruined politically.

Of course, Parliament must satisfy public opinion. That is what elections are for. But Britain should be governed by the rule of law, sound administration and democratic elections, not panic inquiries, retrospective fines and free- wheeling political lynch mobs. Because of the actions not just of MPs, but of their critics too, parliamentary democracy is being undermined. Who is going to pay us back for that?