It was at about this time of year, and the roads were icy. My bike slipped from under me and I rolled on to the pavement. I thought at first I was unharmed, but when I pulled myself up I found my left foot could not support my weight. I phoned a taxi. The accident and emergency department was almost deserted. It was evening, but not late enough for the broken drunks or the seasonal fight victims – goodwill to all men unless they're looking at my bird – to start arriving. I dragged myself over to reception, got logged and hobbled away to the waiting area.

Only one person was sitting there. There were no magazines I wanted to read, so I parked myself two spaces from him in the hope of starting a conversation. He had a number one haircut and tattoos on his neck and knuckles. His hands and face were filthy. He wore a stripey fleece jacket, like the one I once owned, until I lost it. His was thick with grease and soot, and pitted with cigarette burns.

"What are you in for?", I asked. He held up a finger, suppurating, black and yellow, missing its nail. "Blimey. How did that happen?"

"Pig slammed it in his car door."

"Why?"

"Because he's a fucking pig."

"What had you done?"

"Nothing. Fuckers were moving us on."

"Oh. What sort of a vehicle do you live in?"

"Old ambulance."

"So where have you moved to now?"

He looked at me then turned away without answering. I had transgressed. Travellers, as I had found when I'd written about the harassment they faced, were often – and for good reason – wary of telling people much about their lives. We sat in silence for a while, then he abruptly asked if I had a £10 note. Now it was my turn to respond suspiciously. "I don't want it, I just want to show you something." I passed him the note. "Wanna see the queen's bum?". He cropped the queen's face with his filthy fingers until the fold between her jaw and neck looked vaguely like a naked arse. He laughed, showing black teeth, and handed back the note. We relapsed into silence.

I racked my brain for another gambit, then inspiration struck. "Do you have any dogs?" The transformation was remarkable. He turned to me, his face lit up. "Yeah, two. One boy, one girl." "What sort?" "Staffies." "Oh, I love staffies." "These ones, you'd have to see them, it's like they can understand what you're thinking. D'you know what I mean?" "I do know what you mean. What colour are they?" "Brindled. I've always had brindled ones."

I almost fell off my chair. I sat open-mouthed, scarcely able to breathe, trying to regain my composure. I looked at him. It had been – what, five years? – and he had thickened and reddened in that time, but I had no doubt that he was one of them. I looked harder. He was the older brother: the worst one.

A group of us had occupied a piece of land on St George's Hill in Surrey, 70 miles from where we now sat. In 1649, the Diggers had built their settlement there, in the hope of establishing a "common treasury for all". Our aim had been to rekindle interest in land reform. It had been going well – we had placated the police, started to generate plenty of public interest – when two young lads with brindled staffordshire bull terriers arrived in an old removals van.

Everyone was welcome at the site and, as they were travellers, one of the groups marginalised by the concentration of control and ownership of land in Britain, we went out of our way to accommodate them. They must have thought they had died and gone to heaven.

Almost as soon as they arrived they began twocking stuff. A radio journalist left his equipment in his hire car. They smashed the side window. Someone saw them bundling the kit, wrapped in a stolen sleeping bag, into their lorry. There was a confrontation – handwringing appeals to reason on one side, pugnacious defiance on the other – which eventually led to the equipment being handed back.

They wound their dogs up, making them snap and snarl at the other occupiers. At night they roamed the camp, staffies straining at the leash, cans of Special Brew in their free hands, shouting "fucking hippies, we're going to burn you in your tents!"

We had no idea how to handle them without offending our agonised liberal consciences. They saw this and exploited it ruthlessly. Eventually the police solved the problem for us. Most of the cars parked at a nearby attraction had had their windows smashed and radios stolen, and someone had followed their lorry back to our site. As they were led away, my anarchist beliefs battled my bourgeois instincts, and lost.

While the memories rushed back, he carried on talking, apparently unaware that I wasn't listening. At last I managed to master my feelings and show some interest in the puppies he was hoping to breed. And then it dawned on me. He was wearing my coat.

Twitter: @georgemonbiot