‘This is my fucking investigation, Nightingale,’ said Seawoll. ‘I don’t care who you’re currently fucking — I don’t want any of your X-Files shit getting in the way of proper police work.’

‘I can assure you, Inspector,’ said Nightingale, ‘I have no intention of getting in your way.’

Seawoll turned to look at me. ‘Who the hell is this?’

‘This is PC Peter Grant,’ said Nightingale. ‘He’s working with me.’

I could see this shocked Seawoll. He looked at me carefully before turning back to Nightingale. ‘You’re taking on an apprentice?’ he asked.

‘That’s yet to be decided,’ said Nightingale.

‘We’ll see about that,’ said Seawoll. ‘There was an agreement.’

‘There was an arrangement,’ said Nightingale. ‘Circumstances change.’

‘Not that fucking much they don’t,’ said Seawoll, but it seemed to me he’d lost some of his conviction. He looked down at me again. ‘Take my advice, son,’ he said quietly. ‘Get the fuck away from this man while you still have a chance.’

‘Is that all?’ asked Nightingale.

‘Just stay the hell away from my investigation,’ said Seawoll.

‘I go where I’m needed,’ said Nightingale. ‘That’s the agreement.’

‘Circumstances can fucking change,’ said Seawoll. ‘Now if you gentlemen don’t mind, I’m late for my colonic irrigation.’

He went back up the corridor, crashed through the double doors and was gone.

‘What’s the agreement?’ I asked.

‘It’s not important,’ said Nightingale. ‘Let’s go and see if we can’t find this dog.’

The north end of the London Borough of Camden is dominated by two hills, Hampstead on the west, High-gate on the east, with the Heath, one of the largest parks in London, slung between them like a green saddle. From these heights the land slopes down towards the River Thames and the floodplains that lurk below the built-up centre of London.