Yet the most remarkable thing about Jeremy Corbyn’s triumph is that he achieved it by doing exactly what he has always done: talking about the same issues at the same kind of meetings he has spoken at all his life. That 251,000 people — just 0.5 per cent of the electorate — agree with him is not surprising. That he managed to muscle his way to the top of the Labour party is more remarkable.

To fill time while the supporters of a “new kind of politics” wait for the man of the hour, Billy Bragg takes to the rally’s makeshift stage, an open-sided lorry parked on the south side of Parliament Square. “A historic day, is it not?” says the 57-year-old songwriter and fixture of the British Left. The roar from the crowd suggests that they are as excited about Corbyn’s victory as Bragg is.

As Bragg launches into the opening lines of “The World Turned Upside Down” — “In Sixteen Forty Nine, to St George’s Hill a ragged band they called the Diggers came to show the people’s will” — fists and pacifist V-signs rise into the air. The under-thirties tap their feet and nod their heads but it is the over-fifties who seem to enjoy Bragg the most. One veteran with a blue mohican knows more or less every word. Between songs, Bragg muses on the Corbyn win: “Solidarity is not for losers any more.” He tells the crowd that because of Corbyn he will do something he has not done since the early 1990s: attend his local Labour party branch meeting. Corbyn needs our help, Bragg says: “It’s going to be a tough one, this. They’re going to throw everything at him, the press. They’re going to throw his presence here at him. His support for peace . . . his principles . . . his beliefs.” The crowd is treated to another Bragg classic, “Waiting For The Great Leap Forward”, with a special commemorative line added: “The revolution is just a Labour leadership contest away”.

Eventually Corbyn steps up to the microphone, telling the thousands there that he has never seen Parliament Square look “so full, so beautiful and so happy as on this day”. He gives the sort of speech he has always given — “Together in peace. Together in justice” — before Bragg leads the crowd, and Corbyn, in a rendition of the old Left’s favourite anthem, “The Red Flag”:

So raise the scarlet standard high,
beneath its folds we’ll live or die.
Though cowards flinch and traitors sneer,
we’ll keep the red flag flying here.