Usually, Callum had scant patience with Tamsin’s penchant for evolutionary psychology. But right now they were performing, as couples do in company, a pat double act. Callum tucked his hands up under his armpits and capered like an ape until Tamsin pretended to cuff him round the head.

‘Right,’ he said, turning to Chris. ‘Enough of all that. Let me get you a drink.’

This little routine wasn’t wasted on Chris. He had accepted Callum’s invitation out of a sense of kismet: because he barely remembered giving Callum his number, the text message seemed, somehow, to be a call to destiny, a prompt it would be foolish to ignore. His initial resolution to leave Tamsin and Callum in peace had dissolved in a froth of conjecture (was she unhappy with Callum? was Callum unhappy with her? how had she felt, meeting him again after all those years?). Now he was here and he could see the situation for what it was – domestic bliss – his role was very clear. There were no decisions to make, no moral dilemmas to brood over. He would talk to Callum, eat his supper, adore Tamsin from afar, then go back to barracks life.

What he hadn’t bargained on was liking Callum quite as much as he did. Chris would meet his few non-army friends at the weekends for sixteen hours of expensive hedonism before crawling back to Bulford. In contrast, Callum and his compact little flat were, as Chris pronounced loudly over pudding, ‘the peak of civilisation’. It was all wonderful: Callum’s cooking, the canvas photo prints of Moroccan souks and Scottish islands (all Callum’s own work), the complete set of Loeb classics on the homemade bookshelves, the electric drum kit in the corner of the room on which Callum let him mess about and finally, after much protesting, demonstrated a short but breathtaking burst of eight against nine.

‘Our Callum’s something of a Renaissance man,’ Tamsin remarked, drollery a poor mask for her pride.

Best of all, Callum appeared to be fascinated by Chris. He asked question after question about the army, and actually took out a small notebook when they got started on the history of the machine gun.

‘Can you believe it? Gatling, the guy who was basically responsible for the machine gun mark two – after the Maxim, that is – genuinely thought he was saving lives. One soldier kills a hundred times more people, so you need a hundred times less soldiers. I mean, go figure.’

Tamsin watched them as they talked, feeling relieved that they had not, so far, approached the question of her history with Chris. Yet she was also feeling curiously excluded. She had been dreading conversation about Afghanistan or Iraq, two subjects on which she felt herself to be embarrassingly under-informed. But neither Chris nor Callum seemed interested in what she thought.