‘You don’t even have to ask me the right question,’ he said. ‘This information is gratis. I owe you this.’

Cooper played the recording again. The professor sounded in a bad way. Not just eccentric or strange any more, but disturbed. He had seemed a little too close to the edge.

The call had been made well over an hour ago, when Cooper had been talking to Tom Jarvis. But Robertson hadn’t left his phone number. Probably he’d just forgotten, since he seemed so distracted. Cooper dialled 1471 for Caller ID, but a recorded message told him the caller had withheld his number. It was almost as though the professor was still taunting him. I’ve got something to tell you. But you’re never going to be able to ask me what it is. Ah, what a lark. He wondered whether Robertson had permanent number withhold on his phone, or if he’d prefixed his call with 141 specially to make life difficult for Cooper.

Never mind. He had the professor’s number in his book. It wasn’t a problem. But before he rang, Cooper played the message a third time, listening carefully to the voice, trying to judge whether it was sincere, what the underlying emotion was, how unstable the professor might have become. Shaking his head, he dialled the number.

Twenty miles away, in a refurbished Edwardian house on the outskirts of Sheffield, Professor Robertson’s phone began to ring. But the oak front door had already closed, a key had turned in the lock, a car started up in the drive. The house was empty.

And now it was Ben Cooper’s turn to leave his voice on a machine. He was talking into a void.

Most of the restaurants in the High Street were closed on Monday night. The pubs were open, but full of under-age drinkers. At this time, there was nowhere he could comfortably find something to eat except McDonald’s. Oh, well. One Happy Meal wouldn’t ruin his arteries, would it?

Cooper didn’t immediately recognize the staff member serving behind the counter. Perhaps it was the uniform and baseball cap, the cloak of corporate anonymity, that fooled him.

‘You’re Ben Cooper, aren’t you?’ said the young man, after putting through the order and taking his money.

‘That’s right.’

Cooper looked more closely. He didn’t forget faces easily, and this one did look vaguely familiar. Gelled hair with blond streaks, a stubble of beard, a nose that had perhaps been broken once, but mended well. There was something about the eyes, now that he took the trouble to look the young man in the face. Perhaps it was an arrest he’d made some time in the past?

‘I’m sorry,’ said Cooper. ‘I know I’ve met you, but I can’t remember where.’

‘I’m Nick Summers. My dad’s a friend of your brother Matt’s.’