John Rentoul: The new gold standard: Don't blame us, guv, we weren't even there

Sketch: Cameron appeared to purr smoothly, with the moral conviction of someone who has been in power for five minutes

David Cameron slid into the Chamber as smoothly as a Dalek, armed with the most powerful weapon in the universe. He knew that he was invulnerable because he was protected by an anti-politics force field. The Labour Party had been caught out engaging in politics, and it was all before he had been responsible for anything.

So he didn't have to try hard to look prime ministerial. He pitched it low – so low that the Labour benches' early-warning mechanism failed to pick up the incoming. He wasn't accusing them of anything, he said. Jack Straw, the former Foreign Secretary, was quite right to say that Sir Gus O'Donnell's review had found nothing in Straw's statements to the House, or those of his extinct colleagues, that was contradicted by what they said in private.

But, the Prime Minister concluded, mildly: "We weren't really given a complete picture." Labour ministers did not have the power to release al-Megrahi, but they thought it might be convenient if the Scottish government did it. More than that, they thought it might be in the national interest if it happened, regardless of who actually took the decision.

Who would have thought that politicians might want to "facilitate" something that they thought was in the national interest, even if they couldn't make it happen, and wouldn't want to admit what they thought was in the national interest because it meant compromising justice in a hard case? Even if they didn't, in the end, facilitate anything? That sort of thing shouldn't happen in politics, should it, Cameron appeared to purr smoothly, with the moral conviction of someone who has been in power for five minutes and whose hands are clean.

It's a high standard to hold MPs to. Give us the complete picture. Never mind the truth, or just the absence of lies. Tell us all the bits you don't want us to know. Condemn yourself for the compromises you thought of making, the game plans that you considered and rejected.

Cameron's attack on the Labour former ministers was more effective than the synthetic outrage of Malcolm Rifkind, who, having forgotten all the compromises he made while genocide was being practised in the Balkans when he was Foreign Secretary, railed that the "previous government was up to its neck in this shoddy business".

It went straight through Ed Miliband who, like Cameron, was a smooth-faced innocent at the time. There is only one person who can be harmed by those neutron rays in the end, and that is the one in power who has to make hard decisions and now can be held to the new gold standard of parliamentary accountability: "We weren't really given a complete picture." It's going to make Prime Minister's Questions quite long tomorrow.

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