The flat was, at first, as you’d expect: I was led into a short corridor lined with knee-high piles of books, magazines, loose papers and copies of the Guardian and Standard. Like my own home. I knew at once what I’d find in the living room: a wall given over to bookshelves, either Ikea or built-in, wedged with books and decorated with a self-conscious sprinkling of postcards, invitations, photographs, mementoes and so on.

Wrong. At least two upstairs rooms had been knocked through to form Pierce’s living room, which ran all the way from the front of the house to the back, with big windows at each end. This yielded a long side wall that would have been perfect for metres of shelves. But this wall was instead filled with a huge map of London.

‘Map’ is in fact not a useful term to describe what Pierce had made. In the south, Biggin Hill and Purley were at skirting board level; the northern stretch of the M25, where it runs past Waltham Abbey, was at the ceiling cornice. The outer outline of Greater London was just about recognisable, as was the blue vein of the Thames where it widened in the east, and a few exposed green patches at Richmond, Blackheath, Epping and elsewhere. The rest of the city was obscured by a thick aggregation of matter, which lifted and shivered when Pierce threw open the door and walked into the room. It was a layer of sticky notes, index cards and clippings – several layers, from the look of it, anchored by hundreds of coloured thumb-tacks. Further notes encrusted the wall to the left and right of the map; the room also contained four metal filing cabinets and the usual living-space furniture: a three-piece suite facing a telly at one end of the room, a dining table and chairs at the other.

‘Wow,’ I said.

‘The map?’ Pierce said with a pained expression. ‘I’ve been meaning to take it down.’

‘What? Why? What is it for?’

He didn’t answer, and instead waved an arm in the direction of the cracked brown leather sofa. ‘Sit. Sit. Coffee? Tea?’

‘Coffee would be great,’ I said, taking off my coat. I had to move a couple of magazines off the sofa in order to make a place for me to sit: a TLS and a Time Out. But before I sat I remembered the need for colour. Ignore Pierce’s instruction, check out the room. I approached the map.