David Cameron rarely makes the same mistake twice. The Conservative leader’s first reaction to news that Derek Conway, MP, had employed his student son was to issue a statement to the effect that he had been punished enough.

In Giles Chichester’s case he engineered a resignation within 24 hours of the offender’s admission of a breach of the rules and reminded all Conservative representatives of the need to “adhere to the highest standards”.

Insiders seek to explain away Mr Cameron’s initial failure to grasp the gravity of the Conway case, laying the blame on a busy schedule on the January day that the damning Commons report was published. “None of us really realised just how bad it was until we had actually had time to read the details of what he had done,” a Shadow Cabinet minister told The Times.

But there is another reason why Mr Cameron failed to remove the whip from Mr Conway straight away – he wanted to deny Labour the chance to crow over “Tory sleaze”.

Scarred by memories of how financial scandals dogged the Major Government, the Tory leader’s initial instinct was defensive. After the Conway affair he has become convinced that the world has moved on and it is the Government, not the Conservatives, that has the bigger sleaze problem.

Mr Cameron has ridden the wave of public disgust over allegations of abuse of parliamentary allowances by staying ahead of Mr Brown, insisting that he is leading the campaign for greater transparency.

He has required from all Tory frontbenchers details of their employment of family members, for example, and a public report is due at the end of this month. It is not a strategy without risks, however, as the Chichester case shows. Insiders say that Mr Cameron has been monitoring developments closely since the funding arrangement was first reported at the weekend.

His aides were at pains last night to emphasise how “robust” he had been – even in the face of Mr Chichester’s insistence that he has done nothing wrong. With greater transparency and closer scrutiny Mr Cameron might find himself having to be “robust” on an increasing number of occasions. As Tony Blair discovered to his cost, pledges to be “purer than pure” can be hard to live up to.

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