Porter and Styles kicked around a handful of ideas on the way back to the station. If this were any other case, they would have scooped Bolton up, sealed him in a box-like room and verbally sparred until he cracked. That wasn’t an option here, though. Campbell was the gatekeeper to any of that after the dressing-down he’d given them. He wouldn’t be easily swayed on the say-so of a witness who, on reflection, had seen nothing concrete, nothing provable. It would be different if they found her first. If they could find Natasha, go to Campbell with a body … but that brought with it a logistical nightmare. Styles had googled Ruislip Woods on the way to the cafe; they stretched over seven hundred acres. Even with a vague sense of where Mary Locke had led them, they had as much chance of winning the lottery this weekend as they did of stumbling across Natasha.

It was a little after two by the time they parked and headed upstairs. The lunchtime exodus meant most of the other officers had migrated to the cafe downstairs, or headed out for something more edible than the usual in-house offering. The debate in the car had reached a stalemate. Porter wanted to convince Anderson and Whittaker to join them for a speculative poke around in the woods. Styles preferred to chip away at Campbell, hoping Mary Locke’s version of events, as difficult to prove as it was, would convince him to send a canine unit out.

A Post-it note caught Porter’s eye as he dropped into his chair, block capitals scratched deep into the surface, sticky strip barely clinging to his screen. He didn’t recognise the writing but then he noticed it had been signed by its author, Reid, the young officer who had been chasing up footage from the weather station. Porter grabbed the phone and punched in the extension number Reid had scribbled down.

It only rang twice before he answered. ‘Reid speaking.’

‘Reid, it’s Porter. Just seen the note. You got anything for me?’

‘Ah, yes, sir. I’ll be right up.’

Porter went to ask him what it was, but heard the click as the line went dead. He didn’t have long to wait. Reid came bustling in less than a minute later, almost colliding with Styles, who was heading out to the bathroom. Reid held a hand out to Porter, who looked at him blankly at first, waiting for him to speak, until he saw the memory stick.

‘Do you want to tell me what’s on here or is it a surprise?’ he asked, plugging it into a spare slot at the base of his PC.

‘It’s not the best quality, sir, and the delay between frames makes it a bit jerky, but see what you think.’