Hawkins didn’t hesitate, launching herself upwards, grabbing the bar above her head and trying to haul herself up. She brought a leg up, scrabbling for grip against the moist brickwork. But she lost traction, dropping flat against the wall.

‘Ma’am.’ Chris’s voice came from behind her, as she felt hands locking under her left heel, boosting her. With his help, she wrenched herself over the parapet, twisting to roll sideways down on to the far side.

She yelled thanks back at him, spinning towards the noise of screeching tyres, to see the watcher stumble past the slewing vehicle and resume his sprint down the wide avenue opposite the park, still heading south.

‘Sorry!’ she shouted to Chris, leaving him behind as she took off across the wide avenue when a gap in the traffic opened up. The watcher was retreating fast along the right-hand pavement, and Hawkins glanced up at the street sign, conscious that she might need to direct back-up.

Holles Street.

She glimpsed the man as he leapt between two elderly women, driving herself harder when she calculated that getting over the wall had cost her ten yards. She was already overheating inside her winter coat, but there was no time to discard it as she wove in and out of increasingly dense foot traffic, light spilling on to the rapidly darkening street outside a window display full of reindeer and elves.

Then she realized where they were.

She looked up, catching the John Lewis sign. Which meant this road led directly on to one of London’s busiest avenues.

Oxford Street.

She swore, annoyed to have been so focused on the chase that she’d overlooked the fact Cavendish Square was just a hundred-odd yards from the capital’s seething consumer heart, where their quarry clearly aimed to lose himself in the crowds.

The pavement widened again as Hawkins approached the intersection, helplessly watching her target round the corner ahead. She risked a glance behind, seeing Chris thirty yards back.

He looked out of shape.

She reached the junction, skimming pedestrians as she swung right on to the main thoroughfare, forced to slow as the crowds closed around her like liquid, shoppers sliding past in every direction. Blocking her view.

She paused, searching for signs. There was no obvious break in the murmur of a busy pre-Christmas shopping scene. Seasonal music trickled from an unseen source, all but drowned out by the background of myriad conversations and passing traffic. To her right, a few staff members in Santa costumes stood in the John Lewis foyer, handing out leaflets as people came by. Their blithe expressions said they hadn’t just been passed at speed. But he could have pre-empted that; dropped into a casual stroll as he wandered in, maybe even smiled and taken a leaflet. Leaving no trace.