Many years ago I was given a lift home from an Iranian party by a young women who was an up and coming comedian...now famous.

As we approached Kentish Town (where I live) she said, 'Why do all the mad people get off at Kentish Town?' It was a good question then and it's a better one now.

Let me describe them: there's the small woman with the shaven head and no teeth who pushes her possessions around in carrier bags in a supermarket trolley. Very occasionally she rants at something but rarely.

There's the elderly black woman always heavily rouged who never seems to walk, only sits - sometimes in silence watching, but often spitting obscenities at every individual passing by. 

There's the rather upper-class sounding young man with a megaphone who harangues us all as he stands outside Kentish Town station, orating about the various conspiracies behind everything.

And today, as so often, there's the tall middle aged character - lean, rangy in a long coat and beanie - who walks at speed around and around the raised platform where there is a flower stall, a few seats under a glass roof and a van selling coffee. He pushes a monocycle in each hand and talks without pause cheerfully, with excitement, at the top of his voice. 'I have the world's largest collection of knives,' he was announcing as I passed him this afternoon. 'You should see them. There's one for...'

Who should see them? All of us or the person in his head he was addressing? I have never seen him arrive nor depart. He is either there or he is not.

Why do all the mad people get off at Kentish Town? I have no idea.