The Oxbridge International Language School had an unmanned reception desk and, beyond it, a tiny classroom with half a dozen desks. It was empty apart from an overweight man in his forties, vaping furiously, and a couple of exhausted-looking Asian women in their twenties. When they are not conjugating verbs, most of these language students are out working long hours for low pay.

‘You speak English?’ I asked the man, showing him my warrant card.

‘That’s a bloody strange question,’ he said.

‘How many registered students do you have on your books?’ I said.

‘I would need to check online.’

‘Ballpark figure.’

I watched him hesitate.

I indicated Billy.

‘PC Greene here would be happy to check your records, if you find your mind’s gone blank.’

The man licked his lips.

‘One thousand, nine hundred and twenty-seven,’ he said.

I looked at Billy and then back at the man.

‘So the Oxbridge International Language School has nearly two thousand students on its books?’

He nodded, studying the floor.

‘Look at me,’ I said.

He looked at me. I gestured at the handful of desks and chairs.

‘What happens if they all show up at once?’ I said.

‘Well,’ he said. ‘It gets rather crowded.’

‘I bet they all have a student visa, right?’

He looked hurt. ‘Of course!’

I took out my phone.

‘Look at this woman,’ I said.

I showed him the photographs of Rabia Demir and Zlatko Draganov. While he was studying them, I glanced at Billy and I saw that he got it.

This was the way our people had to play it with these language schools. Show them that their cosy little student visa scams would abruptly end unless we got some help with our enquiries.

‘Beauty and the beast,’ the man said, attempting a smile. Trying to bond with me.

‘Just concentrate on the woman,’ I said. ‘Forget the man. The man’s dead. Have you seen her?’

He studied the face in the seven photographs. I gave him the phone and he scrolled through the images himself, more slowly this time, trying to recall.

‘What’s her name?’

‘Rabia Demir,’ I said.

‘Turkish,’ he said. He handed me back the phone and shook his head. ‘We have some Turks but they’re mostly men. And she’s not one of the women.’ He smiled weakly. ‘I think I would remember Rabia.’

I gave him my card.

‘If she shows up, you call me. And if I don’t hear from you, my colleague here will come back to work out how you manage to squeeze two thousand students into a classroom the size of a telephone box.’

He took my card and nodded obediently, but I saw the doubtful light in his eye and I knew exactly what he was thinking.

There are ten million people in this city.

And I was looking for just one woman.

Keith Li called me as we were going down the stairs.