Stephen Butfoy is co-founder of the British Academy of Krav Maga, a form of self-defence whose first principle is, “If you can, run away.” “If they want your watch,” he says, “give them your watch. Don’t wear anything you can’t afford to lose.” He recounts a few recent stabbings in London, full of motivational detail and bird’s-eye observations. (The guy gave the mugger his watch, but then punched him on a reflex. He was stabbed and killed.) I am agnostic on whether or not they’re really, you know, true. The French couple in the class clearly already believe the capital to be a very dangerous place. Defensively patriotic, I want to interject, “Sure, muggers – but also, you know, Maritime Museum” but I’m scared of Butfoy, who is made of iron filings.

“Today we’re going to do knives,” he explains. “Only we’re going to use twigs. There are knives in the bag, but we can’t use them as we’re in a park.” We met at Asics, a sports shop in Oxford Street, at noon, before moving to Hyde Park. Before we start, there’s a lot of running, squatting and press-ups, “because we really need to pressure test you, see how you react when you think you have nothing left to draw on”.

Today we’re acting out a robbery where you don’t just hand over your valuables. The principles are: redirect (the knife); control (also the knife); attack (the person); take away (the knife). Bring your hands parallel to those of the knife-wielder; pretend to take your watch off; smash his hand from beneath so the knife flies out of it (this really works); punch him in the goolies; run away. We practise this diligently with a twig. A gigantic staghound lumbers over and tries to help. It is fun to watch him chase the twig.

I’m not taking this seriously. I’ve imbibed the early lessons – about your most vulnerable stab areas (wrist, neck, inner thigh) – too well, and I wouldn’t engage in so much as a cross word with an assailant, never mind try to punch him in the nads.

“What if there’s someone with you? Now you can’t run.” Butfoy moves to the side of his mugger, grabs the knife hand, locks his own arm. “You lock it, he cannot free himself. All he can do is move your body sideways.” Another unexpectedly true thing. With your free hand, poke him in the eyes, or do a figure of four (twisting his knife hand till he lets go), or punch him in the throat. It’s not a very rigid discipline: basically, if it’s hurting (someone else), it’s working.

Butfoy is demonstrating on the French gentleman, who isn’t thrilled about this, sprawled on the floor, without his twig. “Now are you going to run away?” I ask, worried everyone except me has forgotten the first principle.

“Now I’ve got his knife. I’m going to rob him.”

The hour is up – 20 minutes of cardio, 40 of fighting. Even after the first session, I have a feeling, possibly illusory, of increased physical competency.

Butfoy slips on a watch of outrageous bling. I don’t know enough about him – or watches, for that matter – to know if he can afford to lose it. But I’m pretty sure it won’t come to that.

What I learned

This is the fighting system of the Israel Defence Forces, devised by Imre Lichtenfeld in 30s Czechoslovakia.