‘But she wasn’t then,’ Callum replied. ‘I checked. And why be comfortable when you could be wealthy?’

The more Kate turned over the events of the morning in her mind, the more certain she was that she was right: if Ian had been seduced into working for the Russians, his fragile ego was the cause.

She only broke the silence to tell her companions that she wanted to make a small detour to the home of the former chief of the Service, Sir Alan Brabazon. Julie queried the wisdom of her visiting a potential suspect of their inquiry, but Kate insisted she knew him well enough to pass off the visit as a purely social affair. She dropped Julie and Callum for lunch in Stockbridge, a town on the bank of the Test river, and headed to the old mill-house hidden in the folds of a lush valley south of Winchester.

The electric gates opened automatically to a long and winding track, which led past grazing sheep to a wide gravel parking area in front of a red-brick house that sat sideways on a meandering stream. The garden was immaculate, clipped lawns sloping down to the water’s edge, which was flush with water-lilies and wild flowers. The far end was dominated by a well-tended orchard, a series of raised vegetable beds and an iron and glass greenhouse.

Sir Alan and his late wife had often professed themselves devoted to the garden of their country house.

Kate stepped out of the car. The bright afternoon sun was filtered through the leaves of a beautiful, drooping beech. A table beneath it was covered with lichen, suggesting it was rarely used.

Kate knocked on the door. It was opened by the smiling figure of Mrs Evans, Sir Alan’s housekeeper of many years. ‘Mrs Henderson,’ she said. ‘This is a pleasant surprise. Sir Alan will be delighted.’

‘He’s not expecting me, I’m afraid.’

‘There are some surprises he’ll relish.’ She stepped back to allow Kate into the cool flagstone interior of the hallway. ‘Are Stuart and the children with you?’

‘No. I was just passing on my own.’

‘They’re well, I hope?’

‘Yes, yes.’ Kate wondered if Mrs Evans was aware of Stuart’s treachery. ‘I mean Fiona and Gus are teenagers, so …’ She smiled, as if that said it all.

‘They’re still delightful, I’m sure.’

‘Possibly with strangers.’

Mrs Evans chuckled. ‘Go through to his study. He’s in one of the outhouses, fiddling around with that wretched old MG of his. I’ll go and find him. Would you like coffee?’

‘Yes, please,’ Kate said.

She did as she was instructed, following the flagstones to the far end of the house. The study overlooked the bubbling brook beyond open windows. It was a dark room, with low ceilings, heavy with old beams and many hundreds of books, which filled every available inch of wall space. At one end a desk overlooked a flowerbed at the corner of the garden. It was groaning with files of the kind one used to find in Registry, before the Service had digitized its archives. She could clearly see one marked ‘Top Secret’.