Toby suddenly crouched down at my feet and growled, a proper rat-in-the-kitchen growl that was all business. I looked over and saw a woman gliding towards us across the polished marble. She was slender and dressed like an Edwardian maid, complete with a starched white bib apron over a full black skirt and white cotton blouse. Her face didn’t fit her outfit, being too long and sharp-boned with black, almond-shaped eyes. Despite her mob cap she wore her hair loose, a black curtain that fell to her waist. She instantly gave me the creeps, and not just because I’ve seen too many Japanese horror films.

‘This is Molly,’ said Nightingale. ‘She does for us.’

‘Does what?’

‘Whatever needs doing,’ said Nightingale.

Molly lowered her eyes and did an awkward little dip that might have been a curtsey or a bow. When Toby growled again Molly snarled back, showing disturbingly sharp teeth.

‘Molly,’ said Nightingale sharply.

Molly demurely covered her mouth with her hand, turned and went gliding back the way she came. Toby gave a little self-satisfied snort that didn’t fool anyone but himself.

‘And she is … ?’ I asked.

‘Indispensable,’ said Nightingale.

Before we went up, Nightingale led me over to an alcove set into the north wall. There, resting on a pedestal like a household god, was a sealed museum case containing a copy of a leather-bound book. It was open to the title page. I leaned over and read: Philosophiae Naturalis Principia Artes Magicis, Autore: I. S. Newton.

‘So not content with kicking off the scientific revolution, our boy Isaac invented magic?’ I asked.

‘Not invented,’ said Nightingale. ‘But he did codify its basic principles, made it somewhat less hit and miss.’

‘Magic and science,’ I said. ‘What did he do for an encore?’

‘Reformed the Royal Mint and saved the country from bankruptcy,’ said Nightingale.

Apparently there were two main staircases; we took the eastern one up to the first of the colonnaded balconies and a confusion of wood panelling and white dust sheets. Two more flights of stairs led us to a second-storey hallway lined with heavy wooden doors. He opened one, seemingly at random, and ushered me in.

‘This is yours,’ he said.