In her conference speech earlier this month Theresa May claimed that Labour had now admitted their manifesto at the last election would have costed one trillion pounds. “You heard me right - one thousand billion pounds”, she added, for the benefit of those unsure what one trillion meant. She repeated the claim at PMQs a few weeks ago. I suspect it’s not the last time it’ll get trotted out.

I feel some responsibility, since I am fairly sure this is a result of a couple of lines in a book I’ve just published on the 2017 election. I’m not sure if anyone in Number 10 has read it – but that particular bit attracted the attention of both the Sun and the Mail, and I am sure they read the Sun and the Mail.

Before this becomes established fact, it’s worth noting that this isn’t quite what the book says.

Labour’s boast during last year’s campaign was that their manifesto was “fully costed”. Yet anticipating possible attacks from the Conservatives and a hostile press, one internal Labour email (which I saw when researching the book) highlighted some of the problems with their costings. These included the lack of detail on capital spending, as well as some individual costings that were implausible or entirely absent. They covered almost every area of the manifesto, including welfare, health, education, the economy, transport, policing and prisons. Collectively they came, even conservatively, to billions of unaccounted spending.

The “one trillion” line is a separate point. This comes from one of those involved in the preparation of Labour’s manifesto, who believed that it would have been possible for the Conservatives to have credibly claimed there was £1 trillion of spending commitments in the Labour manifesto. Such a claim would have involved the Conservatives being a bit imaginative, but it would have been – our source felt – at least plausible for them to have done so.

The lack of any serious attacks on Labour’s spending plans came as a constant shock to some of Labour’s staff. When Seumas Milne presented Labour’s election strategy to the party’s ruling national executive committee, he predicted that the main Conservative attack would focus on how “Corbyn would bankrupt Britain”. “It didn’t add up! It didn’t add up!”, said one party staffer, talking about their own manifesto. “That was obvious to anyone who looked in any detail. I just kept thinking, they’ll tear us apart on this. But the attack never came”.

Some of the credit for this goes to John McDonnell, for deciding to publish costings in the first place. This was a gamble, and one that many people at the top of Labour’s campaign did not want to take. Yet McDonnell insisted, and – combined with the lack of any comparable costings on the Conservative side – it helped deflect any Conservative attacks on spending. (McDonnell’s often-used line that the only numbers in the Conservative manifesto were the page numbers wasn’t actually true – but it worked). But it was also because even when those attacks came, Labour insiders couldn’t believe how weak and poorly researched they were. Talking of the once feared Conservative Research Department, one of those responsible for dealing with Labour’s response described his Conservative opponents as “so lazy” and the work as “properly ropey”.

So this isn’t quite the same as Labour admitting their manifesto would have cost a trillion. If the Conservatives had had the rigour they could have identified billions of extra spending; had they had some imagination, they might have been able to inflate it to a trillion. Perhaps the most important thing about this story, though, is that in 2017 the Conservatives lacked both the rigour and the imagination.

Philip Cowley’s The British General Election of 2017 (with Dennis Kavanagh) is published by Palgrave.