There are subjects that unite everybody outside SW1 in revulsion, yet which the Westminster political classes are so utterly used to that they find no need even to explain, let alone excuse. In my experience it is the things that “everyone knows goes on” that are most likely to get people outside SW1 going: what do you mean, ministers will secretly leak their own plans, and then deny it? What do you mean, MPs get food and drink subsidised by taxpayers (40 per cent off)? And what, they might add, do you mean - “everyone knows” that MPs routinely employ members of their family to the tune of tens of thousands of pounds a year at the taxpayers' expense? And it's legal?

By what perverted, insular reasoning do we deem it okay for the plummest jobs in the country - working in the House of Commons, assisting an MP, a foothold on a political career - to be doshed out on the basis of nepotism? Today, in the 21st century, when we have rules for public appointments, open selection procedures, scrutinising committees, how can it possibly be acceptable for an MP to appoint his wife and children to his Commons staff and pay them tens of thousands of pounds of public money each?

This is the element of the Derek Conway scandal that will most resonate, and for longest, with the public: MPs employ their families at the taxpayers' expense? You knew that? And you (the media) never made a big deal about it? Mea culpa.

But pause for a moment to consider the precise allegations against Mr Conway, for even by the standards of parliamentary nepotism he is extraordinary. First, the MP for Old Bexley and Sidcup used part of his staffing allowance (up to £87,000 a year) to pay his son a £11,000 salary, equivalent to a full-time salary of £25,970 a year, while he was on a full-time course at Newcastle University. The MPs who investigated the complaint found not only that it was “inherently improbable” that the son, who “seems to have been all but invisible during the period of his employment”, worked the full contracted hours; they also thought his salary “excessive”, “far out of line with what would have been justifiable”.

But there's worse. Not content with bunging his son £11,000 a year (plus pension), Mr Conway gave him top-up bonuses amounting to: £2,000 in September 2005, £5,000 in May 2006, £1,300 in January 2007 and £1,765.94 in May 2007. (The Honourable Member actually requested £5,000 for this one but was only allowed the smaller amount.) We're up to nearly 45K. Are you angry yet?

The committee of MPs found that the bonuses went “way beyond the permitted ceiling”, a ceiling that Mr Conway managed not to breach when employing other members of staff for the preceding three years. “This arrangement was, at the least, an improper use of parliamentary allowances: at worst it was a serious diversion of public funds.” In any other field, in the private sector, it would be treated as fraud.

Now hark at Mr Conway, in an apology to the House lasting not quite a whole minute. “I will not delay the business of the House by going through the contents of that report, as it is publicly available...”, he began. No, let us delay it. Let us go through the contents again, and slowly: £43,770. “Far out of line... way beyond the permitted ceiling”, “inherently improbable” the boy did the work, “all but invisible”, a “serious diversion of public funds”.

It had been, the MP pleaded, “a very difficult period. The House will comprehend the impact that this matter has on me personally and also on my family...” Ah, poor me. “I have let them down very badly indeed, and no judgment from any quarter could be more harsh than that which I apply to myself.”

No judgment could be more harsh? What about resignation? Prosecution? Jail? The man is unbelievable.

Oh, and it also turns out that the MP employed another son to the tune of £32,000 in his busy undergraduate years - and Mrs Conway too! At a cost of £260,000 in family salaries over the years.

This is nothing like the financial errors that caused Peter Hain to resign, large though those were, let alone the smaller reporting errors of other Labour deputy leadership candidates. By what manner of serious misjudgment David Cameron took almost 24 hours to come to the conclusion that Mr Conway should have the whip withdrawn will doubtless form a paragraph in some future biography of the Tory leader. Mr Conway's position as an MP was unsustainable from the moment the report was published.

But ah! said the BBC, in its first online analysis of the scandal on Monday, “MPs taking on members of their family to work on their staff is not at all unusual. Many have to spend the working week away from home...” And oh, added its political editor the next day, the penalty is already severe compared with those normally handed out by the Commons. Meanwhile, most newspapers buried the story in their inner pages.

Severe? Try prison! Try finding some of Mr Conway's constituents jailed for significantly less benefit fraud, or a mother separated from her children because she couldn't pay a court fine. Then tell me Mr Conway's punishment is severe.

Beyond the Westminster village, these cosy arrangements are shocking. Employ your relatives, employ your mates. Carve up the political goodies. Keep it in the family. It is paternalistic, it is undemocratic and sometimes, as Mr Conway has shown us, it is corrupt. Yet we were so used to it, we thought it unworthy even of comment. All of us who are not MPs have one thing to thank Mr Conway for: he has exploded it into the light.

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