John McDonnell threw the Little Red Book at George Osborne. It was a wonderful moment of parliamentary theatre, and in its way an admirable one.

The present Labour leadership is often attacked by cruel Conservative sketchwriters, and even by cruel mainstream Labour MPs, for not having had a new thought since 1976. And here was McDonnell playing up to this stereotype.

He would draw for support on the works of Chairman Mao, who as it happens died in 1976, at the end of a long and murderous life.

It was, we may say, sporting of McDonnell to do so. He had perhaps decided, as he prepared this most difficult speech, that he might as well be hanged for a sheep as a lamb.

The shadow Chancellor realised there could be some mileage in mocking “Comrade Osborne” for becoming so chummy with the Chinese Communists, and for promoting the nationalisation of British assets as long as it is the Chinese state which purchases them.

From this realisation, it was but a small step to help Comrade Osborne to understand the Chinese, and to think how amusing it might be to quote from of the sayings of Chairman Mao.

So McDonnell quoted a typically plonking remark which ends with the words, “we must not pretend to know what we do not know”.

But the whole art of being a Chancellor is to pretend to know what we do not know. One has to take a plausible view about what is going to happen for the next five years.

Which is what Osborne tried to do in 2010. He said that within five years he would get rid of the deficit. This he has failed to do: and McDonnell quite reasonably reminded us that Osborne has failed to do it.

But at this point, Osborne’s critics find themselves faced by an almost insuperable difficulty. Almost all of them think he was right not to get rid of the deficit so quickly: to have done so would almost certainly have done more harm than good.

Or to take another example: Osborne today abandoned his tax credit reform. In full view of the Opposition, he executed that most difficult manoeuvre, a U-turn.

But what was McDonnell to say about that? For here too, he thought Osborne had done the right thing. He tried to claim credit for making Osborne turn round, but the attack soon became a kind of compliment to the Chancellor for having the wit to see when a policy has become counter-productive.

Osborne’s speech was mostly rather dull and relentless, but none the worse for that. It amounted to an apologia for the last five years, and a relentless recitation of promises for the next five. He offered something to almost everyone: women’s charities, Alastair Campbell, Winston Churchill, the NHS, the police, people who can’t afford a house…

And then McDonnell gave him the gift of the Little Red Book. Osborne picked the book up and made a joke about it being McDonnell’s “personal signed copy”. The Chancellor added that “half the shadow cabinet’s been sent off to re-education camps”.

How sleek and self-satisfied the Prime Minister looked during this stage in the proceedings. David Cameron was at his most pink and prosperous, for he knew the Opposition had just made a laughing stock of itself.