I had a look around while Zach and Lesley followed us up. We were in a space with the mean dimensions of a living room in a council flat, although the ceiling was low even by those standards. Low enough for me to scrape my helmet if I didn’t watch it.

‘Watch your head, darling,’ Zach told Lesley as she came up.

At first I thought the walls were panelled with dark wood in the Victorian fashion, but I quickly realised that the colour was wrong, too pale. When I rapped the panels with my knuckles there was the unmistakable ring of ceramic. But when I brushed them with my fingertips I felt wood grain, and mingled with that was tobacco smoke, beer and whisky. I looked at Nightingale, who was frowning as he too touched the wall. He caught me looking and nodded. The air was still, musty and dry.

‘We need to get on,’ he said and what with Kumar, Reynolds and the last two CO19 officers it was getting a bit tight in there. There was only one exit, a doorway framed with more fake ceramic wood.

Like the well behaved coppers we were, we let the CO19 officers go first. After all, there’s really no point bringing them if you insist on standing between them and any potential targets.

The doorway led to a long corridor lined not with fake wood panelling this time but with nasty mauve wallpaper. If I needed any further indication that the Quiet People didn’t have much of a colour sense then that wallpaper was it. At evenly spaced intervals were hung what looked like empty picture frames. Nightingale put a hand on each of the CO19 officers’ shoulders.

‘Quickly and quietly, lads,’ he said.

Off we went, just as quietly as you’d expect from people wearing half a ton of various types of gear between us. Safety tip: wading trousers – not built for stealth. We pulled up short of where the corridor ended in a T-junction.

‘Which way now?’ Nightingale asked Zach.

‘I don’t know,’ he said. ‘This wasn’t here last time.’

‘I really wish you hadn’t said that,’ said Lesley.

I was thinking of Space Hulk myself, but there are some things you don’t say out loud in front of other police.

Nightingale didn’t hesitate. He gestured at the CO19 officers and one went left and one went right. Nightingale went with one and I went with the other.

There was a single gunshot, astonishingly loud in the confined space. I threw myself back round the corner but Nightingale yelled, ‘Hold your fire.’

There was a long moment of silence in which I took the opportunity to pick myself up.

‘I believe that was a warning shot,’ said Nightingale. ‘Peter, if you’d be so good as to ask Mr Palmer to come forward.’