‘If it’s so good, I suggest you use it to find out who killed your mother.’ He said a hurried goodbye, and walked out along the endless echoing corridor back to reception. One of the receptionists he’d seen earlier gave him a cheery greeting. ‘Did you enjoy your stay?’ she called out as he passed.

‘I’ve only been here an hour,’ he replied. As he exited the sliding doors, he could hear gales of laughter behind him. He’d been made a fool of, but he wasn’t sure how. And how on earth did that woman know so much about him?

Research Intelligence Officer Rob Townsend caught up with Gillard as he was arriving back at Mount Browne. ‘Just been having a look at Mrs Roy’s laptop, and there is some video footage on there that you might like to look at.’

‘Okay. Make it quick. I’ve been summoned to see Rigby in a few minutes.’ Meetings with the chief constable always made him nervous, and she was a stickler for punctuality.

Townsend led Gillard into his office, where almost every surface was covered with pieces of computer equipment: laptops, hard drives, and standing aside on the floor the precious data kiosk, which saved them hours on extracting the data from phones. The young detective constable was constantly teased about being an anorak, but his skills had been crucial in an increasing array of investigations. Electronic evidence was playing more and more of a central role in prosecutions, and given the explosion in online data, being able to access and interpret it quickly was vital.

Townsend clicked on his own PC in which a data stick was sticking out of a USB port. Video software opened, with slightly choppy and grainy footage of a suburban semi-detached house viewed from inside a car parked on the other side of the road. The date subscript was 18 May the previous year. The front door of the house opened and a woman that Gillard recognised as Morag Fairburn emerged, turning to speak to somebody within. The voice-over was clipped and technical. ‘Okay, 0745 hours, here is some action. Miss B leaving house. Embraces target at the door.’ The camera zoomed in closer. Gillard could clearly see the target was a dishevelled Harry Roy, in a bathrobe. ‘Miss B walks to garage.’ She slid up the door, went inside, and a short while later a black BMW 3 series emerged. ‘Vehicle B emerges, Miss B at the wheel.’ The car turned right out of the drive past the surveillance vehicle and out of camera shot.

‘There’s more,’ Townsend said. He slid the cursor along until the sound chart spiked, indicating more narrative. The next video was almost identical, but on a different day.

‘They’re all pretty much like this,’ Townsend said.

‘This is pretty innocent stuff surely.’ Gillard asked. ‘Why on earth would Mrs Roy have a private detective watching her own son?’

‘I think I have an answer for you. I dug up an old email from her to the agency, Walker Chapman. She’s looking for evidence that her son is having what she calls an illicit affair.’