They had kept a safe distance of at least three cars behind, but she showed no signs of being aware of a tail, let alone trying to evade them. She drove west through Stanmore. Porter wondered if she was heading to their rendezvous early and just wanted time to compose herself with whatever she wanted to talk to them about, but she went past the turn-off and on towards Northwood.

After another five minutes, she turned south towards Ruislip. Porter was beginning to think she had spotted them after all, and was leading them on a pointless loop back towards their intended meeting place. They were nearing the village, tucked in two cars back behind a red Mini Cooper that made Porter think of The Italian Job, when he saw her signal left onto a street marked Reservoir Road. Neither of the two cars between them followed suit, so after Porter made the turn, he eased off the accelerator to avoid catching her up.

The straightness of the road ahead made him even more cautious. As much as he didn’t want to risk losing her around any bends, there was now nothing but a hundred yards of road in between them.

Styles sensed his hesitation. ‘Let her go. It’s a dead end up ahead.’ He tapped the screen on the satnav.

Porter slowed to fifteen miles an hour as they passed the gate on their right that lead to a lake called Ruislip Lido. Originally a reservoir built to feed the Grand Junction Canal back in the 1800s, it was known nowadays for its artificial sandy beach, speckled with towels and picnic blankets in the summer months. It was bare today, though, a couple strolling arm in arm its only occupants.

Up ahead he saw her brake lights winking as she came to a stop, and he quickly pulled over to the kerb. He reached across Styles to open the glove box, and then remembered he hadn’t replaced the compact binoculars he’d broken several months back. He made do with leaning forward in his seat and narrowing his eyes in a squint as the stick figure in the distance exited the Range Rover, its lights giving a rapid double flash as she locked it. Instinct from Porter’s previous life in the army registered it as two dots, the letter ‘I’ in Morse code. He smiled at the memory.

He heard the click of a camera shutter, and turned to see Styles resting his iPhone on the dash. He had zoomed in far enough to be able to identify Mary Locke, but not so much that the picture had surrendered to grainy vagueness. Porter nodded his approval at the improvisation.