‘Yeah, well, that’s what you get on the NHS,’ said Macdonald.

‘It’s not a hospital scar,’ the doctor said. ‘This wasn’t done in an operating theatre.’

When the doctor saw that Macdonald wasn’t going to explain the origin of the wound, he pulled out a stethoscope and listened to his breathing. He examined his throat, then had him sit down while he checked his reflexes with a small metal hammer. The brief physical examination over, he asked Macdonald a dozen or so medical questions, ticking off boxes on a chart on the clipboard. Macdonald answered all in the negative: he was in perfect health.

‘Drugs?’ asked the doctor.

‘No, thanks.’

The doctor smiled thinly. It was obviously a joke he’d heard a thousand times. ‘Do you have a drugs problem?’ he said.

‘No,’ said Macdonald.

‘Alcohol?’

‘The odd pint.’

‘Ever been treated for depression? Anxiety?’

‘I find a five-mile run usually gets me sorted.’

The doctor stood up. ‘That’s the lot,’ he said. ‘You can get dressed now.’ He pulled back the curtain and walked away. A prison officer Macdonald hadn’t seen before was standing by the cubicle holding an armful of bedding.

As soon as Macdonald had pulled on his forensic overall, the officer thrust the bundle at him. ‘These are yours, then,’ he said, in a lilting Welsh accent. ‘I’ll take you to the remand block.’ He was a small, balding man with a kindly face.

Macdonald looked down at his bedding. There was a thin pillow, a pale green pillowcase, a green sheet and a brown blanket.

‘Don’t hang about,’ said the officer. He already had his key in his hand and unlocked a barred door with the minimum of effort. He stood to the side to let Macdonald through, then followed him and relocked the door. Macdonald glimpsed the key. It was like no other he’d seen before, no rough edges, just small discs set into the metal strip, which he guessed were magnets, impossible to copy.

The officer walked him through another barred door that led on to a corridor covered by CCTV cameras. It stretched for several hundred yards and was deserted. Their footsteps echoed off the cream-painted walls as they walked towards a door at the far end. The officer unlocked another barred door and took Macdonald up a flight of metal stairs to the first floor. There, two guards were standing in a glass-sided cubicle. One was tapping at a computer terminal; the other was drinking a can of Coke.

The Welshman pointed for Macdonald to stand where he was, then walked into the cubicle. ‘Got a mystery man for you,’ he said, handing over the file to the guard with the Coke, a tall, broad-shouldered man with bulging forearms.

He scanned the file. ‘Okay, thanks, Taff,’ he said. He dropped it next to the computer. ‘I’ll take it from here.’

The Welshman walked away, whistling softly.