‘I can only draw what he saw, Max,’ said the sketch artist. ‘And Professor Hitchens doesn’t know what he saw.’

He wrung his hands. ‘I’m sorry, I’m sorry.’

‘How long have you been working on this?’ I said.

The sketch artist glanced at her watch. ‘Couple of hours?’

I shook my head and stared at Hitchens with disbelief.

‘Tell me one more time what happened,’ I said.

He brushed his nicotine-stained fingers across the great sweaty dome of his head.

‘You entered the culvert. When you didn’t come back out, I called out to you because I was worried. I can’t recall exactly what I said . . .’

‘You said, Detective and then you said Max – and that was when my assailant let me go and did a runner . . . Then what?’

‘I was trying to text you,’ Hitchens said. ‘I was looking at my phone when I heard footsteps in the water – moving very fast. I thought it was you – and I looked up and . . . I stared straight into his face just before he sent me flying. And that’s where you found me. Sitting on my backside.’

Tara Jones came in and began clearing her desk. She saw me looking at her and nodded, sending a torrent of black hair forward. She pushed it off her face and began packing away the hardware that was scattered all over the workstation she had used for the last month.

I patted Hitchens on the arm. ‘You saw him. You did, Adrian. Briefly – very briefly – but you saw him. But it’s just out of reach.’

‘I don’t understand why I can’t recall . . .’

‘It’s the shock of violence. Even if you were used to it, this would be hard. And you’re not used to it. You just had one of the worst experiences of your life. Don’t feel too bad. I looked at him too and I didn’t see a thing.’

‘But you were in darkness and I was in blazing sunshine . . .’

‘You want me to try some software?’ the sketch artist asked.

In recent years there has been a move to improve facial composites with what we call a feature recognition system – the witness is shown complete faces until a final image emerges. But I shook my head.

‘Pencil and pad still works best,’ I said. ‘Just keep trying,’ I told Hitchens.

‘I am . . .’

‘Try harder.’

I walked over to Tara Jones. I had worked out by now that I needed to be directly facing her before I started talking. She looked up at me and smiled politely.

‘You’re leaving,’ I said.

‘My contract with the Met was for a month. And the month’s over. So . . .’

‘Thanks for your help with the tapes.’