The barista behind the counter of the coffee shop in the middle of the subway tells me she can’t put up Billy’s poster without consulting her manager, and he’s not in until 5 p.m. The queue at the sit-down coffee shop just yards away is too long to even contemplate talking to a member of staff, so I drop a pile of leaflets on the table nearest the door instead. As I hurry through the subway towards platforms thirteen and fifteen I scan everything I see – posters, free newspaper racks, walls, doors – but they’re graffiti-free. If Billy did tag the train station he didn’t do it down here.

I stop short when I reach the top of the stairs to the platforms. There’s a wreck of a building on the opposite side of the tracks. It’s the derelict sorting office, now little more than a rectangular slab of concrete with gaping holes where the windows used to be. As I watch, pigeons flutter in and out but it’s not the birds that catch my eye. It’s the graffiti daubed all over the building. There are high walls, topped with barbed wire, surrounding it but that wouldn’t stop Billy, not if he was determined to put his mark on it.

‘Excuse me, madam.’ A hand grips my shoulder and I spin round to find myself face to face with a tall man in a luminous yellow waistcoat and a black peaked cap.

‘British Transport Police,’ he says, glancing at the bundle of paper in my hands. ‘It’s been reported that you’ve been distributing material to members of the public. Can I see your licence or badge, please?’

‘Licence?’ I step away from the yellow line on the platform edge as a train pulls into the station and the overhead announcer reports that the 11.30 a.m. train to Paddington is standing at platform thirteen. ‘What licence?’

‘You need a licence from the council to distribute leaflets at this station. There’s a fixed penality of eighty pounds or a court-imposed fine of up to two thousand five hundred if you haven’t got one.’

‘But … I … I don’t know. I came with my mum. She’s the one who got the leaflets printed and I’m sure she’s got permission for us to—’

The doors to the carriages open and, as the passengers disembark, I’m distracted by a fracas further up the platform. There’s a small crowd of people around one of the doors and a man is shouting at someone to stop pushing in.

And then I see him. Tall, slim, in a baseball cap and a black Superdry jacket, shoving his way to the front of the queue.

‘Billy!’ I fling the leaflets away from me and sprint up the platform. ‘Billy! Billy, wait!’