Illicit. It was all there, in that word. As a detective, it had been her job, and therefore less fun, to poke around in other people’s business. There was also the novelty factor. Charlie could barely believe it but it was true: in all her years as a DC and then a DS, she’d never straightforwardly tailed somebody one-to-one. Doing so – even better, to her own sister – had given her a rush of joyful adrenaline that was hard to describe. She’d found herself hoping it would take many more months to get to the bottom of Liv’s deception.

‘Has Gibbs ever mentioned a Nikhil Gulati to you?’ she asked Simon. ‘Liv hasn’t to me.’

He didn’t take the bait.

‘That’s his name – the male half of the couple we saw Liv and Gibbs with in Cambridge that time. Shall I tell you how I know? When Liv got on the Cambridge train, I did too. I watched her make a beeline for the first coach – first class; only the best for Liv! – and made sure to get on a few coaches along, in standard. It was a train for King’s Lynn, but it stopped at Cambridge and I had a strong suspicion that was Liv’s destination, so that’s where I got off, concealing myself in a swarm of business suits. Liv was just ahead of me, at the ticket barrier. Again, I was lucky. She didn’t see me.’

‘Very lucky,’ said Simon.

They’d arrived at the police station car park. Charlie drove past two narrow spaces and pulled into one with no car on either side of it. She turned off the engine.

‘I followed her out of the station. There’s a turning circle immediately in front of it, and a taxi queue to the right. I expected Liv to go for a cab, but instead she headed left and went to stand at the edge of the circle. I thought, “She’s waiting for someone to come and pick her up,” and sure enough, a few minutes later a Skoda pulls up, the window comes down and there he is – Nikhil Gulati. I didn’t know that was his name then, obviously. I’m jumping ahead.’

‘Can the rest wait?’ asked Simon, opening the passenger door. ‘I’ve got work to do.’

‘You know what I think? You’re secretly pissed off that I’m the one who’s found all this out and not you. No, it can’t wait! The rest won’t take long. I’d have told you sooner if you hadn’t been obsessing about that old woman and her Asda van.’

‘I want to interview her myself,’ Simon murmured, his eyes glazing over.