‘Kumar and I,’ I said to keep him happy, ‘will descend while Lesley and a couple of bods from the Murder Team will hang about on the tracks just in case.’

‘Just in case of what?’ asked Nightingale. ‘What are you expecting to find?’

‘I don’t know, tramps, trolls, sentient badgers – you tell me.’

‘Not trolls,’ said Nightingale. ‘They prefer riverbanks, particularly spots overshadowed by stone or brick.’

‘Hence the stories about bridges,’ I said.

‘Precisely,’ said Nightingale. ‘As far as I’m aware, nothing unusual lives in the tunnels, or the sewers for that matter. Although there are always rumours, colonies of vagrants, tribes of navvies that have become trapped underground and turned cannibal.’

‘That was a film,’ I said.

‘Death Line,’ said Nightingale, surprising me. ‘Starring Donald Pleasence. Don’t look so shocked Peter. Just because I’ve never owned a television doesn’t mean I never went to the cinema.’

Actually I’d always thought he sat in the library with a slim volume of metaphysical poetry until the Commissioner called him on the bat-phone and summoned him into action. Holy paranormal activity, Nightingale – to the Jag mobile.

‘The cinema of David Lean – yes,’ I said. ‘Low-budget British horror films – no.’

‘It was filmed just around corner from the Folly,’ he said. ‘I was curious.’

‘Any rumours that weren’t made into a film?’ I asked.

‘An old school-chum of mine called Walter once tried to convince me that any system, such as an underground railway or indeed the telephone network, could develop genius loci in the same fashion as the rivers and other sacred sites.’ Nightingale paused to negotiate a tricky knot of traffic as we got off the Harrow Road.

‘Was he right?’ I asked.

‘I couldn’t say,’ said Nightingale. ‘Once he got going I never really understood more than one word in ten, but he really was terribly bright so I’m at least willing to entertain the possibility. Certainly if a Scotsman introduced himself to me as the god of telephones I’d be inclined to take him at his word.’

‘Why a Scotsman?’

‘Because of Alexander Graham Bell,’ said Nightingale, who was obviously in a whimsical mood that night.

We did the strange square Bayswater one-way system and turned up Queensway, which had opted for Christmas lights this year. Many of the shops were open late and the pavements were crowded with shoppers. The weather had obviously concentrated the pre-Christmas rush into a mad panic.

‘Have you found time to buy your presents yet?’ asked Nightingale.

‘Already sorted,’ I said. ‘Got my mum’s’ – an envelope full of cash because my mum is definitely not of the thought that counts school of Christmas giving. ‘And I found a mint 1955 original Easy Geary LP for my dad.’