I opened the flap and had a look. Inside were two metal cylinders the size of aerosol cans but much heavier. They were white with No. 80 WP Gren. stencilled around the body. At the top there was a spring-release trigger held in place by a large metal pin. I’m not a military buff, but I know a hand grenade when I see one. I looked at Nightingale, who gave me an irritable wave.

‘Put them away,’ he said.

I closed the satchel and settled it gingerly over my shoulder.

Nightingale turned back to Frank. ‘Are your people ready?’ he asked.

‘Two appliances on standby – just in case.’

‘Good man,’ said Nightingale. ‘We should be done in about half an hour.’

We got back in the Jag and Nightingale directed me across the station bridge and down a couple of identical streets until he said, ‘This one here.’

We found a parking spot round the corner and walked the rest of the way.

Grasmere Road ran parallel to the railway and looked utterly normal, a string of detached and semi-detached houses built in the 1920s with mock-Tudor façades and bay windows. There was nobody about, the kids were all at school and their parents were at work and we kept the pace casual, at least as casual as I could manage with a pair of grenades banging against my hip. Anyone watching would have taken us for a pair of feral estate agents out marking their territory.

Nightingale made a sudden left turn through the gate of a particular house and headed for the wooden door-sized gate that blocked access to the side passage. Without slowing down he thrust his right arm, palm forward, at the gate and with a tiny sound the lock popped out of the wood and clattered onto the pathway beyond.

We stepped through the open gate and stopped in the blind spot. Nightingale nodded at the gate, and I propped it closed with a big terracotta flowerpot. There was still soil in the flowerpot with a shrivelled black stalk poking out. I checked similar pots lined up on the sunny side of the path; they were all dead too. Nightingale stooped down, grabbed a handful of soil and crumbled it beneath his nose. I followed his lead, but the soil smelled of nothing, sterile, as if it had been left on a windowsill for too long.

‘They’ve been here a while,’ said Nightingale.

‘Who has?’ I asked, but he didn’t answer.