I'd just finished work, and I had met a friend with the intention of going to the pub and drinking things. It was just after 7pm, and we headed down Wharfdale Road in London's King's Cross. As we walked and chatted, we passed two lads fiddling with a bike attached to a tree with a cable lock, in a manner not becoming of bike owners. One was holding a motorcycle, the other was in a hoodie. Not that people in hoodies don't own bikes, you understand. The whole scene just looked wrong.

We stopped a short distance away and turned back to give them another look. One of the men was shaking the bike vigorously. We both headed back towards them, without verbalising what we were going to do if they were indeed pilfering. As we approached, one pulled out bolt cutters and, with a quick snip, freed the bike.

As we walked back and passed them, they tried to look all innocent – a tough look to pull off in the circumstances – assuming that we'd keep walking. I turned back to face them, placing my left hand on the bike's seat.

I said "You're trying to steal this, aren't you?" which in hindsight was a bit direct.

The hoodie grabbed the bike from me and tried to mumble something about it being theirs, but the motorcyclist broke from this narrative to say: "Leave it, I'm going to knock you out." With that they were gone, the hoodie on the stolen bike, the motorcyclist on his motorbike.

We were left feeling pretty foolish and impotent. We looked around, not really sure what to do next. There were at least half a dozen onlookers, who were all staring at us, but hadn't said anything or got involved. Though to be fair, the exchange probably took less than a minute.

A man outside the nearby pub rang the police, and as we headed off to another pub we discussed what we should have done – I wasn't wearing my glasses, so wouldn't be confident of identifying them (the hood and the helmet making things more difficult – though my friend said the larger one "resembled an ugly James Corden"), and stupidly we didn't take the motorbike's registration number.

Also, we thought that if we confronted them they would just run away, because that's what we would have done in the situation, were we ever to be tempted by a life of bike crime.

At the second pub, we related the story to the bar staff, who suggested we leave a note for the cyclist to let them know what had happened, which we did. They also told us that they'd witnessed two people – the same two? – attempting to steal a bike from outside the nearby Tesco. The same snip of the wire cutters, the man leaping on the bike … only to crash straight to the ground. The bike's owner had taken the pedals off.

The unlucky bike owner – a Frenchman – emailed us back, expressing a kind of urbane surprise: "I was not expecting such things as, thought I, the bike was watched by CCTV."

An estimated 80,000 bikes are stolen in London every year, and as rehabilitated bike thief Omar Aziz told the bike blog earlier this year, CCTV is no deterrence. Nor, it seems, is the presence of passers-by, or ham-fisted confrontation attempts by Guardian-reading types. Two good locks – as suggested by the London Cycle Campaign – or crafty pedal removal seem the only ways to ensure safety.

The owner is going to ring later for more details, and I will attempt to explain, and apologise for not having done more. With so many bikes being stolen, I am curious to know what others would do – or have done – in the same situation.