Boris Johnson knows how to hold his tongue. I was reminded of this valuable but often overlooked quality in the Foreign Secretary while watching a preview of Brexit: A Very British Coup? which will be shown tonight at 9.00 p.m. on BBC Two.

At the start, we see Johnson out on the campaign trail, acting as auctioneer at a cattle market in Gisburn, in Lancashire. As he leaves the ring, he is asked by Patrick Forbes, who wrote and directed the programme: “Enjoying it, Boris?”

To this Johnson replies, with great good humour: “Yuh, how could you not?” And as Forbes proceeds to relate,

“That’s my first, and indeed last, question to Boris for quite some time. Leave spinners take umbrage at such a probing inquiry.”

So in this enjoyable and often illuminating documentary about the Brexit campaign we hear at frequent intervals from Gisela Stuart, Andrew Bridgen, Liam Fox, Sir Alan Duncan, Sir Nicholas Soames, Crispin Odey, Jacob Rees-Mogg and Nigel Farage.

But Johnson is for most of the time as silent as a Trappist monk, and certainly takes no impromptu questions from journalists. He provides good pictures.

What self-discipline! While giving people the impression that he is the most open, affable, talkative fellow in the world, willing to have a chat with anyone he happens to bump into on the campaign trail, he says no more than the five words quoted above to the chap making the documentary.

Even under the most severe provocation, Johnson very seldom answers back, or not in any identifiable way. The last time I can remember him doing so was in 1998, when he protested in the Spectator, under the headline “I was stitched up”, about the nasty surprise he got the first time he appeared on Have I Got News For You.

 But even then, the quarrel was soon made up, and he was quickly back on friendly terms with the show’s regulars, Ian Hislop and Paul Merton.

This summer, I updated my life of Johnson, in order to give an account of the amazing convulsions which occurred between 23 June – the day of the referendum – and 13 July, when Theresa May became Prime Minister and to general astonishment, installed Johnson as Foreign Secretary.

One of the reasons why that comeback was possible was that after Michael Gove had announced, also to general astonishment, that he had “come reluctantly to the conclusion that Boris” was unfit to be Prime Minister, and had instead decided to run for the leadership himself, Johnson did not answer back.

The same phenomenon was observable during the referendum campaign. Johnson denounced Barack Obama for coming over here and telling us how to vote, but once the President had made some excellent points in reply, Johnson let the subject drop.

And when, during the debate held on ITV on 9 June, Amber Rudd, for Remain, launched two carefully scripted and rather amusing attacks on Johnson, once again he had the self-discipline, or good sense, not to retaliate.

So Johnson could not long after become one of Rudd’s closest colleagues, and be entrusted with diplomatic  relations with the United States, without embittered wrangles having to be lived down.

Not everyone will regard this forbearance as admirable. Those who demand (quite unrealistically, in my view) that our politicians should behave at all time with complete candour, may choose to see in his behaviour a kind of cunning insincerity.

But a refusal to engage in vendettas is surely desirable in almost any line of work outside the mafia. And the ability to hold his tongue makes Johnson a more formidably professional politician.