Think of a number. Any number. Jeremy Corbyn had thought of a number. He had thought of the number 10,000. That was how many new policemen and women Labour were promising to put on the beat if they won the election. He’d show the Tories he too could be tough on law and order.

Think of a number. Several numbers. The shadow home secretary, Diane Abbott, had thought of several. Unfortunately all of them were the wrong ones as she tried to explain Labour’s policing commitments to Nick Ferrari in an interview on LBC radio.

Ferrari had got things rolling by asking how much 10,000 new officers would cost. Not the trickiest of opening questions as this was precisely the one she had been sent on his show to answer, but still it caught Abbott totally off guard.

“Well ... erm ...” she dithered. Why was this man asking her about policemen? Still, it would be rude not to say anything. But it was tricky. Up till now it hadn’t occurred to her that policemen actually got paid. Like real money. She’d always rather imagined they were all doing voluntary work. “If we recruit the 10,000 police men and women over a four-year period, we believe it’ll be about £300,000.”

Beneath Diane Abbott's police funding gaffes, Labour's numbers make sense Read more

“What are you paying them?” asked Ferrari who was just beginning to realise the interview could be comedy gold. Better even than Natalie Bennett at the last election.

Abbott scratched her head. Hadn’t she just told him what the policemen would be getting? £30 per year sounded more than enough to her. If they didn’t like it they could always go and work in a Nike sweatshop in the Philippines for twice as much. Some people didn’t know how lucky they were. But sensing she might have miscalculated a little, she went back to check her maths.

Radio silence.

“They will cost ... they will, it will cost, erm, about ... about £80m,” she said eventually having just done some long division on her 13 fingers.

“How do you get to that figure?”

Longer radio silence

“We get to that figure because we anticipate recruiting 25,000 extra police officers a year at least,” Abbott declared triumphantly. She was sure Corbyn wouldn’t mind her having arbitrarily decided to recruit an extra 15,000 police officers.

Play Video 3:46 Diane Abbott's error-filled LBC interview on police funding – video

Ferrari went away to do his own sums while suggesting Labour was promising to pay for the extra officers out of capital gains tax reversals that had already been promised to other policies. Abbott was outraged. None of Labour’s policies had been meant to be taken as if they were set in stone and going to be implemented. Rather they were illustrations of all the different things people could have if they won the Generation Game. Kettle, microwave, cuddly toy ...

By now Ferrari had worked out that £80m was going to put the new police recruits on a starting salary of £8,000. Was that what she had in mind? There followed a long silence as Abbott mulled this over. On reflection, it did sound rather a lot.

Longest radio silence.

“We’re talking about, erm, a process over four years,” she reminded him, though she did worry if £2,000 wasn’t still a little too generous.

“Has this been thought through?” asked Ferrari. Yes, of course it had. Just not by Abbott. Or the person who had let her loose on the airwaves.

“The figures are that the additional costs in year one when we anticipate recruiting about 250,000 policemen will be £64.3m,” Abbott blundered on, happy to make up yet more figures as she went along.

“250,000 more policemen?” observed an incredulous Ferrari, not even bothering to ask where the £64.3 million had come from

“And policewomen,” Abbott snapped tartly. Sexist pig. Typical of a man to overlook the important contribution women could play in defending the country from crime.

“You’re recruiting 250,000?”

And his problem was? Ideally she’d have liked to go the full half-mill but she had been told it would have been hard to find any more people with no previous criminal convictions.

But Abbott was not to be distracted. She’d set out to embarrass herself and she was going to make sure she finished the job properly. “We’re recruiting 2,500 and possibly another 250.” Could even be just 25. Sometimes Diane could be like the wind. Ferrari, meanwhile was still trying to work out where the 250,000 policemen – yes, and women – were going to come from.

“I think you said that, not me,” Abbott insisted, before rattling off several other figures. £139.1m. £217m. Who knows? One of them might even be right. She sometimes found it all too easy to forget she had amnesia.

Abbott still wasn’t quite finished as she nipped over to Good Morning Britain to complete the car crash. There weren’t any circumstances in which Corbyn would ever launch a nuclear strike, she told Piers Morgan confidently. None whatsoever. Back in his office, Corbyn was beginning to have a change of heart on nukes. With friends like Diane, who needs enemies?