On the right of the courtyard was the Red Tower, built in brick with green copper-topped turrets and ornate stonework balconies shimmering in the sunlight. To the left, a second vast tower in white stone with four further turrets was linked to the first by a grand frontage of stone-mullioned windows that spanned the full height of the building, reminiscent of Cambridge colleges.

At the main entrance, above an imposing porch, was carved the motto of the family who built the place: Plutot mourir que changer. Vic, with his schoolboy Latin, translated it as ‘rather die than change’, which he thought particularly inappropriate for the work they were doing here, these days: creating change which would, they hoped, save lives.

The afternoon he first arrived, just a month before, had felt positively surreal. The long train journey after a night of little sleep and the meeting with that striking young woman with the flame-red curls; the arthritic old bus that transported them from the station, and the ferryman with a hook where his hand should be. And then his first stunning sight of the Manor. He felt as though he’d entered a world of make-believe, like Alice going down the rabbit hole, and he expected at any moment to wake up and discover that it had all been the oddest dream.

His astonishment had only increased as they’d entered the building through the heavy oak door and into the wood-panelled hallway, passed through the lounge with French doors and views over the estuary into a great hall with, he’d been cheered to see, an upright piano. Frank had introduced him to a man called Johnnie – tall, older, rather avuncular – who led him up a wide staircase leading to a maze of corridors and rooms on the first floor.

‘This’ll be your room, Mr Mackensie. Fifty-four. Not the most luxurious of accommodation but no doubt you’ll be used to it after Cambridge. Make yourself at home. Bathroom’s thataway. Tea’s at four in the Green Lounge. Can you remember how to get there?’

Vic nodded, though he wasn’t at all certain. It would be easy to get lost in this place. He unlocked the door and peered into the cell-like room. This must have been part of the servants’ quarters, and the lowliest servants at that. It was a curious shape, rather dark, with two narrow windows high in the walls, furnished with a cheap plywood chest of drawers and wardrobe, two chairs and two double-tiered metal bunk beds. There was no sign of other belongings, so he assumed – and hoped – that he might actually have the room to himself.

After making up the bed he lay back and tried to gather his thoughts. It had been a whirlwind few weeks: the interview with a couple of stern fellows in an anonymous Whitehall building seemed to go well, and even though he’d barely understood the purpose of their questions he had tried to answer them as honestly as possible. Then, almost by return, came a letter of acceptance with instructions to use his travel warrant on a certain date and a specific train. He looked up Felixstowe on the map and discovered that it was practically the most easterly point of the British Isles, right on the coast and miles from anywhere. Whatever could Watson-Watt and his team be doing in such a remote place?