‘I don’t think so. He didn’t sound like he knew the place well.’

‘Did he have paperwork? A file?’

Marsh considered this.

‘I don’t remember seeing any.’

‘Was the interview filmed?’

‘Not to my knowledge.’

‘Anything else strange that you’ve noticed? Break-ins? Computer viruses?’

‘Nothing.’

We finished our coffees.

‘Does the codename Catalyst mean anything to you?’ I asked.

‘No.’

It had been a long shot. But people always know more than they think they do; they have seen or heard things that mean nothing to them but hold the keys to understanding a situation.

‘What was Joanna working on?’

‘A research group, I think.’

‘Researching what?’

‘I don’t know.’

‘When was it set up?’

‘Mid-2014.’

After Ukraine, I thought. Russia in Crimea. It had been a febrile time in the intelligence service.

‘Targeting anyone in particular?’

‘I have no idea.’

‘Who else in it?’

‘I don’t know.’

‘Why not?’

He looked around, lowered his voice.

‘Strict isolation from all other units. They even had their own canteen.’

‘Housed where?’

‘The old house. Building D.’

‘How many in the building?’

‘Hard to say. It had a separate entrance.’

So within the most classified of government sites they had created an enclave of enhanced secrecy. That usually meant the technology involved was sensitive, or the ethics. Often both.

‘You must have some idea of what was going on in there,’ I said.

‘I don’t know. She joked it was seeing into the future.’

‘Forecasting? Pattern analysis?’

‘Maybe.’

I struggled to imagine Joanna cooped up with software all day, no matter how advanced it was. Tom’s questioning by unspecified security officers worried me. The security concerns could be real or they could be part of a discrediting campaign. Six and GCHQ collaborated on an ‘effects’ unit. Its job was using dirty tricks to make life impossible for those it chose to destroy, and that ranged from the manipulation of bank accounts to dropping child abuse images onto someone’s hard drive. You didn’t want them turning against you.

‘What’s your hunch?’ I said to Marsh.

‘Maybe the program they were working on predicted something which they didn’t want getting out. Maybe she saw too much.’

He said it deadpan. I tried to disguise my scepticism. Still, if I could keep Marsh out of the hands of security, he was a potentially useful ally. I recommended he set up a new email account for contacting me, one that disguised IP addresses.

‘Joanna told me you’d help,’ I said. ‘If anything happened.’

‘Really?’

‘You were one of the few people she trusted. See what you can find.’

I walked back to my car and checked a satellite view of Shefford, locating Building D according to Marsh’s description. They had partitioned the building off from the rest of the grounds. New trees had been planted, alongside more robust fences. Upper windows and skylights had been blocked.