They crossed a short, wind-blown patch of open ground to the main building and entered the busy circular concourse. Luke had been there before, as a serving captain in his SBS days, but it never failed to strike him how impossibly young everyone looked. As they strode along, there was hardly a suit to be seen – in fact, nearly half the people he was looking at were wearing trainers. That’s applied mathematicians, linguists and codebreakers for you, he supposed.

‘Can we offer you both a coffee? A chai latte, perhaps, or a green tea?’ suggested Pullover, steering them towards a Costa Coffee kiosk.

A Costa? At the heart of GCHQ? Over there he could see a Greggs and, beyond that, a Starbucks. To Luke, this place felt more like a Californian campus.

‘Luke Carlton! As I live and breathe!’ an Ulsterman’s voice boomed from somewhere behind them, and he turned to see a broad-shouldered man with a scraggly beard. Luke had no recollection of ever seeing him before.

‘Conor Beattie,’ he introduced himself. ‘We haven’t met, but, hey, nice work on that rescue op in Iran!’ He looked Luke up and down as if committing his image to memory. ‘I was one of those trying to keep track of your movements from here. Have to say, you’re a hard man to keep up with. Anyways, must dash. Things are hotting up a bit across the water in the Province.’ Luke nodded politely, somewhat embarrassed by this effusive encounter. He wondered how many more unseen people in this building had been quietly tracking his movements every time he went live on an op.

‘One of your many fans?’ remarked Jenny, with a raised eyebrow, when the Ulsterman had moved out of earshot. It was Luke’s turn not to answer.

They rode up in the lift, then went along a narrow walkway, carpeted in red and grey, past rows of men and women working away at their screens. It could have been a call centre, a media newsroom or, at a pinch, a City trading room. Except that these analysts were working on largely classified intelligence, tracking anything from North Korean military hacking centres to proxy servers used by ISIS jihadists, from Chinese cyber hackers to internet troll farms in St Petersburg to American child-porn pedlars hiding in the recesses of the dark web.

A large sign welcomed them to the Events Management Centre. ‘GCHQ Never Stops’, it read. How original, thought Luke. He wondered if some management consultancy firm had pocketed a small fortune to come up with that one. They met up with the Norwegian team as they clustered around a bank of screens, talking in low voices. As the introductions were made, Luke hoped none of them would make the connection between him and their dead colleague, Kristian Berge; he decided he wouldn’t mention the subject unless they did.