‘Well that’s the question, isn’t it? Proving something? Trying to get him worked up? Taunting him? Punishing him? Trying to inject a bit of excitement into their dull marriage? Every Thursday, just a bit of unexplained time.

‘He’s a twitchy bugger, and he’d swallowed the bait all right. It was driving him mad. He was sure she was meeting a lover once a week, that her friend Maggie was covering for her. He’d tried following her himself, but he was convinced that she went to bingo on those occasions because she knew he was watching.’

‘So you told him the truth?’

‘Yeah, I did. He didn’t believe me. He got very worked up and started shouting and screaming about everyone being in a conspiracy against him. Refused to pay my bill.

‘I was worried he was going to end up doing her an injury, which was where I made my big mistake. I phoned her and told her he’d paid me to watch her, that I knew what she was doing, and that her husband was heading for breaking point. For her own sake, she ought to be careful how far she pushed him. She didn’t say a word, just hung up on me.

‘Well, he was checking her mobile regularly. He saw my number, and drew the obvious conclusion.’

‘That you’d told her he was having her watched?’

‘No, that I had been seduced by her charms and was her new lover.’

Robin clapped her hands over her mouth. Strike laughed.

‘Are your clients usually a bit mad?’ asked Robin, when she had freed her mouth again.

‘He is, but they’re usually just stressed.’

‘I was thinking about John Bristow,’ Robin said hesitantly. ‘His girlfriend thinks he’s deluded. And you thought he might be a bit… you know… didn’t you?’ she asked. ‘We heard,’ she added, a little shamefacedly, ‘through the door. The bit about “armchair psychologists”.’

‘Right,’ said Strike. ‘Well… I might have changed my mind.’

‘What do you mean?’ asked Robin, her clear grey-blue eyes wide. The train was jolting to a halt; figures were flashing past the windows, becoming less blurred with every second. ‘Do you – are you saying he’s not – that he might be right – that there really was a…?’

‘This is our stop.’