‘I have many masters,’ I said, which caused Madame Teng, when it had been translated, to snort with annoyance. I felt it then, that catching on the edge of my perception, as when Nightingale demonstrates an exemplar forma to me, but different. And there was a brief smell of burning paper. I took an instinctive step backwards and Madame Teng smiled with satisfaction.

Lovely, I thought, just what I needed at the end of a long day. Still, Nightingale would want to know who these people were and as police you always want to come out of any conversation knowing more about them than they do about you.

And, being police, you’re totally used to being considered rude and impolite.

‘So are you two from China?’ I asked.

Madame Teng stiffened at the word China and launched into half a minute of rapid Chinese that Robert listened to with an expression of amused martyrdom.

‘We’re from Taiwan,’ he said when his employer had finished. She gave him a sharp look and he sighed. ‘My employer,’ he said, ‘has a great deal to say about the subject. Most of it esoteric and none of it relevant to you or me. If you’d be pleased to just nod occasionally as if I’m recounting the whole tedious argument about sovereignty to you I’d be most grateful.’

I did as he asked, although I had to restrain myself from stroking my chin and saying ‘I see’.

‘What brings you to London?’ I asked.

‘We go all over the place,’ said Robert Su. ‘New York, Paris, Amsterdam. My employer likes to see what’s going on in the world – you could say that is her raison d’être.’

‘Which makes you what? Journalists? Spies?’ I asked.

Madame Teng recognised at least one of those professions and snapped something at Robert, who gave me an apologetic shrug.

‘Madam Teng asks you once again – who is your master?’

‘The Nightingale is his master,’ said a voice behind me.

I turned to find a stocky black woman in a strapless red dress cut low enough to show off broad muscled shoulders and cut high enough to reveal legs that could do an Olympic-time hundred metres without taking off the high heels. Her hair was shaved down to a fuzz and she had a wide mouth, flat nose and her mother’s eyes. I was caught in a wash of clattering machines, hot oil and wet dog. The cold didn’t seem to be bothering her at all.

Madame Teng bowed, properly, as well she might given she was in the presence of a goddess – that of the River Fleet no less. Robert Su bowed lower than his employer because he had to, but I could see that he didn’t understand why.

‘Hello Fleet,’ I said. ‘How’s tricks?’

Fleet ignored me and gave Madame Teng a polite nod.