‘Sorted?’ asked Lesley as I climbed gratefully into the warm interior.

‘Not exactly,’ I said and told her about the second tracker.

I fished out the thermos flask, yet another Folly antique, a khaki cylinder the size of a shell casing, and poured myself a coffee. Lesley was equally sceptical about us being tracked by CTC, but for different reasons.

‘They don’t need to track us. If they want to know something they’d just phone us up and ask. And if MI5 wanted to know something they’d just call CTC who would call us and ask,’ she said. ‘I think it’s the FBI.’

‘All the FBI has to do is ask Kittredge and he’d ask us,’ I said.

‘But we might not tell Kittredge,’ said Lesley. ‘Not to mention we know Agent Reynolds bent the rules already by following you.’

Lesley went quiet and I paused with the coffee halfway to my lips.

‘Go on then,’ I said.

‘Why do I have to do it?’ asked Lesley.

‘Because I went out last time,’ I said. ‘And I’m still freezing.’

Lesley snarled but she got out of the car and while I finished my coffee she checked for bugs. She was back inside in less than two minutes with another identical GPS tracker.

‘Voilà,’ she said and dropped it into my palm. The casing was freezing – it must have been attached for ages.

‘Agent Reynolds,’ I said.

‘Or somebody else,’ said Lesley. ‘That we don’t know about.’

I twirled the rectangular box in my hand. If it had been set up like ours, then it was probably programmed to send a signal if we started moving. Chances were if I deactivated it now the operator wouldn’t notice until she, or possibly a mysterious they, pinged it to check its operating status.

‘Should I fry it?’ I asked Lesley.

‘No,’ she said.

‘You’re right,’ I said. ‘Because if we destroy it they’ll know we know but if we keep it we have the option of feeding whoever it is false information. We could put the tracker on a decoy vehicle and send them on a wild goose chase or we could use it to set up a sting—’

Lesley snorted.

‘We’re the police,’ she said. ‘Remember? We’re not spies, we’re not undercover and we’re conducting a legitimate investigation that’s been authorised at ACPO level. We want them to follow us so we can identify them, call for backup and arrest them. Once we have them in the interview room we’ll be able to tell who they are by what kind of lawyer turns up.’

‘My way’s more fun,’ I said.

‘Your way’s more complicated,’ said Lesley. She dug her finger under the edge of her mask where it itched. ‘I miss being a proper copper,’ she said.

‘Take it off,’ I said. ‘No one’s going to see you here.’