I found Nightingale in the manageress’ office. He was leaning against the wall, arms folded, as he watched a DC from Intelligence triaging the papers on the desk and packing them into evidence boxes that looked remarkably similar to those the Faceless Man had used to store his goodies in. I hoped they didn’t get them mixed up.

I asked where Guleed had got to.

‘Finishing the interview with the manageress,’ said Nightingale. ‘She seemed to think she’d get more out of the poor woman without me hovering in the doorway.’

‘You know she’s demi-fae,’ I said.

‘So Sahra explained.’

‘You don’t seem very surprised.’

Nightingale shrugged.

‘If Abdul ever gets his wish and finds a reliable . . .’ He frowned. ‘What does he call it – a genetic marker?’

Determining whether there was an actual genetic basis to being a fae was one of Dr Walid’s research priorities. I’m pretty certain that his keenness to employ Dr Vaughan had come from a desire to have more time to pursue it.

I confirmed that Nightingale was right and it was indeed a genetic marker. Although, of course, it was all much more complicated than that, genetics-wise. It always is.

‘Should he ever find his marker,’ said Nightingale, ‘and conduct his survey, I believe he will find that fairy blood is far more widespread than previously assumed.’

And most of them passing, I thought, like Wanda the manageress.

‘None of the items listed on eBay have been sold yet,’ I said. ‘So they must be stored somewhere.’

‘Indubitably,’ said Nightingale. ‘I think we must assume that our tricky fox has a hideaway he hasn’t told us about.’ All of Reynard Fossman’s last known addresses had already been searched over the weekend as well as a few likely lock-ups that had, as Nightingale admitted, quite tenuous connections to the man. Not to mention that we still hadn’t found the antique Renault 4 GTL that, according to the DVLA, was registered in his name.

‘It would be nice to find the Mary Engine,’ I said. ‘It could be the only original difference engine in existence. We could flog it to the Science Museum.’

‘You’d have to fight Harold for it,’ said Nightingale.

Postmartin took his role as archivist very seriously.

‘But it’s not a book, is it?’ I said. ‘That means we get first dibs. Do you know if they made any more – did anyone at the Folly have one?’

‘There was always a rumour that Babbage had worked on a mechanical device of some kind for the Folly,’ said Nightingale. ‘One which might have had applications in the practise – but it was just a rumour.’

‘Was there anything about Ada Lovelace?’ I asked.

Nightingale gave me a funny look.

‘Byron’s daughter?’ he asked. ‘I’m not sure I understand the connection.’

‘She worked with Babbage on the difference engine,’ I said.

‘In what capacity?’