I know the exact moment, the day and the time, the Conservative 2017 General Election campaign came off the tracks, because I saw it happen in real time.

It was the day the manifesto was launched. That morning I was out canvassing in a key marginal seat in south London, held by the party. All morning I had the same response on the doorstep. “I like our MP and I like Mrs May. I’ll be voting for them.”

While we were out on the doorsteps, the manifesto was being launched. We went to lunch satisfied by the morning’s work. Starting again in the early afternoon we sensed a big change in the response we were receiving. Now it was “we like our MP, but the Prime Minister has said this morning she wants to take away our home doesn’t she.” By the evening, half a day and two main news bulletins (1pm and 6pm) later it was “I’m not voting for her, sorry because I like our MP, but…. .” And that was it. Over. For the remaining weeks in that and the twenty other marginal constituencies, I visited during the course of that campaign it was the same thing. The speed at which the mood changed was extraordinary, but change it did and decisively so.

Steve Howell, former BBC journalist turned Jeremy Corbyn advisor and election strategist, sets out in Game Changer – Eight Weeks That Transformed British Politics – how the campaign looked from Labour’s perspective. It is a fascinating story, given that Mr Corbyn has struggled to control his party, despite winning two leadership elections and that by any previous standards Labour was not a notably cohesive fighting force.

Here, however, is the story of how a smallish group at the top of the Labour Party, committed to Jeremy Corbyn, were able to achieve – once the Conservatives had made the manifesto misstep. It was that misstep that gave Labour its chance. Without it, they would have struggled to make the impact they did. That misstep and the weakness of the Liberal Democrats enabled Mr Corbyn’s and Labour’s voice to be heard. Never was the old saying it’s not oppositions that win elections, but the government’s that lose them truer than in 2017.

Howell’s well written and smartly produced book understandably lays claim to more credit than is deserved in winning round a sceptical public, but Labour’s success in attracting, and keeping, younger members and voters is worth noting.

Above all this book is a lesson in how political campaigns are about luck and opportunity as much as anything. Preparation and planning on their own will not deliver victory, but they are a prerequisite to being able to exploit the luck that sees your opponent falter, which then provides the opportunity. This is what Howell and his colleagues did in 2017 and how they did it is well worth reading about.

‘Game Changer – Eight Weeks That Transformed British Politics’, Steve Howell, Accent Press, £15.99