Prime Minister’s questions is the only weekly parliamentary event which regularly fills the chamber. It has developed into a rowdy, raucous occasion, detested by the Speaker John Bercow but enjoyed by everybody else.

For years Prime Minister’s questions involved two 15-minute sessions twice a week, on Tuesdays and Thursdays. But in 1997, the new Prime Minister Tony Blair changed it to one 30-minute session a week, on Wednesdays. Mr Cameron has stuck to this formula.

When television arrived at Westminster, women MPs, for some reason, were advised to wear bright red. So on crowded PM question days, the Commons looked like a sea of grey, interspersed with clusters of love lies bleeding. Margaret Thatcher defiantly stuck to royal blue.

TV also spawned the “soundbite”. Prime Ministers became aware that a carefully-phrased, “sexy” expression was their only chance of getting on the television evening news bulletins.

Leaders of the Opposition are allowed six questions during the 30-minute period, which often occupies half the allotted time, and thus depletes the time available for back-benchers to intervene.

The first combatants in July 1961 were Harold Macmillan and the Opposition leader Hugh Gaitskell. Macmillan always appeared laid back, but in truth he was desperately nervous, admitting that he felt physically sick before each session.

Harold Wilson accused Sir Alec Douglas-Home of being the “14th earl”.

Sir Alec hit back by describing him as “the 14th Mr Wilson”.