‘It’s worth a read,’ I said. ‘I have nothing for you, I’m afraid. Ramil Terzi says hello. He’s on good form.’

Stevenson laughed.

‘House arrest must agree with him.’

We skirted the pond, turned onto a footpath that led into the Heath. Into the trees. Stevenson stopped and crouched, as if to bag dog shit, and we both checked the path behind. No one. When the darkness had closed around us we sat down on a bench. He wrapped the leash around the arm.

‘Joanna’s a bit AWOL.’

‘What do you mean?’

‘Definitely not working for us any more, it seems. Stopped six months ago. Not clear why or where she’s gone.’

‘How can no one know?’

‘This is just off a quick check, a personal check. Not many people are being cooperative. Seems she left under a bit of a cloud. Type of cloud unknown. Out of contact since May.’

‘She was at Shefford.’

‘Yes.’

‘I popped in on Tom Marsh.’

‘What did Tom have to say?’

‘He was questioned about her. And about me. There’s been surveillance on me since I returned to London.’

This raised an eyebrow.

‘You think it’s us?’

‘Hard to tell. What do you know about Building D at Shefford?’

‘Nothing. Shefford is as much of a mystery to me as to anyone else.’

‘Anything codenamed Catalyst that you’ve come across?’

He shook his head.

‘New tech in development? New software?’

‘You name it. But I’m not the man to ask about that. I’m still waiting for my pen gun. I can have a sniff, Elliot. But you know what Shefford’s like.’

I got my phone out.

‘I received an encrypted file three days ago. I believe it came from Joanna. It contained this; the footage is computer-generated.’

Stevenson watched the footage twice in silence.

‘It’s not real?’

‘No. There’s something odd about the way we physically interact. That’s not my body. And the box at the back, there’s no shadow. See how the vase casts a shadow but not the box?’

He looked again, nodded.

‘You think she sent it to you?’

‘It was sent using a system only she knows about. No one else could have done it.’

‘Is there any more?’

‘No. I’ve never been in that room. I’ve been in Kazakhstan twice, but not for many years.’

‘Has Joanna?’

‘Not when I last saw her. That was a few months ago. I had three missed calls around the same time I received the video – from a landline in Kazakhstan.’

Stevenson stared at the screen. I could see professional appraisal. He was looking for signs of provenance.

‘The reflection’s good.’

‘The whole thing’s impressive.’

‘Do you know the other man?’ he asked.

‘No.’

‘The location?’