Daniel Stenner’s expression made Bolton look over his shoulder, expecting to feel a heavy hand grabbing at him. Nothing. Only a forklift nosing out from an aisle fifty yards behind him. He was level with Stenner now, right by the door. It was still closed in spite of Stenner’s hand pumping the handle up and down. Even Locke looked uncharacteristically concerned. Then he knew why Stenner’s eyes were wide in alarm, the first syllables of a curse on his lips.

No time for pleasantries, Bolton moved shoulder to shoulder with Stenner, a tap by his standards, but the contact sent Stenner stumbling sideways. He grabbed at the handle. It turned, the door moved. An inch of daylight, no more. A metallic clang, dull and hollow like an old church bell. He pushed again, sounding the gong for a second time. Again, shoulder to the door, grunting as he threw his full weight into it. He may as well have been throwing himself against the wall rather than the door.

He stopped, breath ragged through snarling lips, squinting through the narrow gap. What he saw made sense of his failure. Vertical grey ridges, a shade darker for being so close, the echoing chime as the door struck.

‘Fuck,’ said Bolton, drawing out the vowel.

‘What? Can you budge it?’

‘Some dozy bastard’s gone and parked a bloody container against it.’

Bolton slapped a palm against the door, regretting it as it sent another chime bouncing down the aisles.

‘Where now, then?’

‘Plan B.’

‘Which is?’

Bolton’s face split into a maniacal grin. ‘I’ll let you know when I figure it out.’

Porter was so focused on the aisles that he didn’t see the fire exit in his peripheral vision until he was past it. He checked his run and bounced back towards it, trying the handle. It turned, but the door stopped after a few inches. He put his eye to the crack, seeing metal draped in shadow, daylight lurking just out of reach. No way anybody left through here.

Styles moved a few paces ahead, signalling the all-clear. First Whittaker, then Palmer followed suit, the world’s smallest Mexican wave. Porter looked up ahead, breathing short and shallow. There couldn’t be more than six or seven more aisles to check. He sensed the end of the chase was around the corner, quite literally.

He moved quickly, expecting to see Bolton charging towards him round every corner, but met with disappointment after disappointment. Empty, all except for the third from last. A man in his forties, standard-issue luminous vest and hard hat balanced on his head, watched them nervously as they ran past.

Porter rounded the final turn, looking the full width of the building, seeing nothing but the other officers appearing a split second later. He swore under his breath and grabbed his handset.

‘This is DI Porter. No sign of suspects inside. What’s happening out front? Over.’