Harriet Harman’s joke did not come off. She was nevertheless right to try it, for one day it might come off.

The Labour leader assured the House that David Cameron is not really in charge. As in a P.G.Wodehouse novel, this seemingly grand figure is unable to make even the most minor move without the say-so of an astonishingly efficient underling. Wooster is under the thumb of Jeeves.

Harman has two problems. One is that Cameron does not look very much like Wooster. He shows signs of being able to think for himself.

Her other difficulty is with her choice of Jeeves. Boris Johnson (Con, Uxbridge and South Ruislip) lacks the self-effacing genius and preternatural calm displayed by that figure. He is not a butler, but one of those whose role in life is to be butled upon.

So when Harman asserted that on the great question of where to put a new runway, “it looks like the Prime Minister has been over-ruled by the Member for Uxbridge”, no one could quite believe her.

People nevertheless wanted to believe her. They very much  liked the idea that, as she said of Cameron, “It seems he’s in a holding position over Heathrow and Boris won’t let him land”.

Could it be true that Boris was even now manning an anti-aircraft gun in Uxbridge? On visiting Uxbridge during the general election, we glimpsed a magnificent Second World War bunker, from which Boris could even now be directing the defence of west London.

He could certainly not be seen doing anything to defend his constituents in the Chamber of the House of Commons. There the fight was led at PMQs by Zac Goldsmith (Con, Richmond Park and North Kingston) , who accused Sir Howard Davies, author of the report proposing the building of a new runway, of starting work three years ago “with a conclusion”.

Goldsmith hoped Cameron would behave better than Davies. Cameron refused to say anything, for he claimed that otherwise he would expose the Government to the risk of judicial review. He nevertheless announced in a calm tone that the decision would be made by Christmas.

The more we see of Cameron, the more we think what an excellent Jeeves he would make. With mellifluous courtesy, he defused a number of attacks from backbenchers.

But when dealing with Dennis Skinner (Lab, Bolsover), mellifluous courtesy does not work. The Beast of Bolsover snarled at Cameron for failing to get subsidies for coal miners, and Cameron replied that it was good to see Labour “in full voice cheering on Jurassic Park”.

This was a kind of compliment to Skinner: after all these years, it is still worth insulting him.

Jack Lopresti (Filton and Bradley Stoke) attacked the Government for failing to maintain defence spending at two per cent. Cameron treated him too with mellifluous politeness.

But the strain on the Tory Jeeves of dealing with his recalcitrant backbenchers must be considerable, and is no doubt one reason why he said during the general election campaign that he does not intend to go on and on.