‘I, er … that is, I have something I need to tell you,’ said Robertson. ‘I hope you don’t mind my calling you at home, but you gave me your number. And, well, there is something …’

There was a pause on the recording. Cooper found himself listening for background noises, the way he’d listened to the tapes of the mystery caller. But there was nothing. No traffic, no voices raised in the opening verse of ‘Abide With Me’. Only the faint whisper of the professor’s breathing, slow and uncertain.

‘Strangely, it’s the one thing we never discussed,’ said Robertson. And now there was a hint of his old self again, the man Cooper had spent so much time listening to. Just a suggestion, but it was there – a sly, ironic taunting that had become all too familiar.

He moved towards the answering machine, thinking the message had ended. But not quite. There was one more thing the professor had to say.

‘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.