So why the criticism of Prevent? “You need to look at who is making it. It is often being criticised by those who are apologists for terrorism, who for their own political ends say it’s ‘spying’ on the population.”

Strong words, but a reflection of just how high this front-liner in the fight against terrorism believes the stakes to be for all of us. “When I started as head of counter-terrorism in 2011, the numbers of operations we were engaged in were much smaller. Prior to the advent of Isil in 2012-13, it was a relatively small number of British Muslims who were going to join al-Qaeda and, therefore, much easier to monitor them.”

What changed was the creation of the so-called caliphate in Iraq and Syria. “The way in which it was done was a masterclass in marketing. It managed to entice as many as 30,000 Muslims to go there to join, from between 85 and 100 countries around the world, according to some estimates. The brainwashing is very powerful and so terrorism has gone from a niche activity to a volume business. We are dealing with high numbers of extremists.”

In such circumstances he cannot, he says, offer any copper-bottomed guarantee that the sort of incident imagined in The Attack won’t ever happen in our cities. “You can’t ever be totally 100 per cent confident you can stop terrorist incidents happening. You are making judgments all the time and you know if you get them wrong, the consequences are devastating.”