‘After realising it must have been Gail who drove towards me in the Land Rover today,’ Perez went on, ‘I wondered if she’d killed her husband in the same way. If he’d had an affair and Gail had killed him out of revenge. But I believe that was just a tragic accident. She loved the bones of him.’

I thought Gail was being so strong, he thought, and all the time grief was eating her away inside. The idea that Sandy might leave her too and she might have to sell the farm that held so many happy memories was enough to tip her over the edge.

‘Talk me through it, Jimmy,’ Anderson said. ‘Tell me what really happened to Anna Blackwell.’

‘The whole thing was planned in advance,’ Perez said. ‘Gail asked Lucy to the farm for a sleepover and suggested that the two women might meet up on the same night. Gail said that Sandy could babysit so she’d be happy to go to Anna’s house. The note that I found was Anna’s reply. She probably sent it home in Grace’s schoolbag.’

‘How did Gail know that Sandy and Anna were having a fling?’ Anderson took a long drink of beer.

‘Sandy told her that he planned to break off his engagement. He explained that he’d fallen for Anna.’ Perez paused and went on to describe how Anna had been killed. ‘On the night of the 10th, Gail left the girls with Sandy and drove into Stonebridge. I think she parked round the corner from the teacher’s house because Anna’s neighbour didn’t see or hear a car.’

Perez imagined the scene. Anna had prepared the house. She might not have felt well enough for a full spring clean, but she’d bought flowers to cheer the place up, and she’d put wine in the fridge to chill. She’d have pictured a friendly chat with the woman who might one day become her sister-in-law.

‘Gail would only have had one glass of wine,’ he said. ‘She’d have explained that she couldn’t drink because she was driving. At one point she went upstairs and crushed the pills she found in Anna’s bedroom into a powder. Everyone in the village knew Anna was taking antidepressants.

‘That was when Gail made her big mistake. The note from Anna was in her pocket and it dropped out on the dressing table. Without the note we would never have been able to prove what had happened.’

Jimmy Perez paused for breath and looked at Anderson, who was listening intently.

‘Gail went back downstairs,’ he continued. ‘Anna would have been quite tipsy at that point. She wasn’t used to drinking. It wouldn’t have been hard for Gail to tip the crushed pills into her wine without Anna noticing. They must have dissolved very quickly. She waited until Anna fell into a deep sleep, then she went into the kitchen, washed out her own glass and put it away. I’m sure you’ll find traces of the drug in the unwashed glass.’