‘An ordinary three-volt lithium button cell battery, exactly the same as you might use in an electronic keyfob or a watch. I can’t confirm the battery life yet, but the manufacturer says an hour. More than enough for the calls made so far, anyway.’

‘So we can’t even hope that he’s going to run out of batteries,’ said Hitchens.

But Petty just smiled as she put away her dictation machines.

‘I think there was a particular reason you recorded the trial message yourself, Liz,’ said Fry. ‘You wanted to make the point that our caller could be a woman, didn’t you?’

‘I’m afraid so. We wouldn’t want to start out with closed minds, would we?’

Ben Cooper leaned back on his desk, wondering when it would be reasonable to phone the hospital again. He’d only just come in, and was waiting for Fry to finish talking to the two support officers. It looked as though she was making sure they knew who was boss.

‘Well, keep trying, Ben,’ she said when he explained the outcome of his visit to Ellen Walker. ‘You’ll get a better result, given time.’

‘You think so, Diane?’

‘Mrs Walker was obviously misled by a superficial resemblance. These facial reconstructions are an art, not a science – no matter what the experts might try to tell you. It doesn’t matter whether they’re done by hand or on a computer. A lot of it is guesswork.’

‘Yes, as a matter of fact, that’s exactly what –’

‘So it’s hardly surprising that you’ll get a false hit now and then. Just put it down to experience. And, like I say, keep trying.’

‘Right,’ said Cooper. ‘Keep trying.’

‘What else have you got there?’

‘I’ve had the dental mapping done by the odontologist. Now I need a dentist who can match the chart to his records.’

Cooper had found a copy of the postmortem dental chart on his desk. Most of the dark areas where work had been done seemed to be in the sides of the mouth, in the molars and pre-molars. The front teeth were almost free of fillings, and were described by the odontologist as ‘regular’.

‘I wish it was as easy as they make it look on TV,’ he said. ‘Like all we had to do was enter details for any set of teeth into some huge database and get an instant identification.’

Fry was no longer listening, but Gavin Murfin looked up from his desk.

‘You mean it isn’t like that?’ he said. ‘The BBC has been lying to me, then.’

Cooper remembered the moments that Ellen Walker had spent staring at him while he tried to recover from his surprise at hearing Audrey Steele had been cremated. All he’d been able to think of to do after that was to ask her for a recent photograph of Audrey.

‘You mean, from not long before she died?’ she’d said.

‘Preferably.’

‘I wouldn’t have one normally. Nothing since we were in our twenties, anyway. But her mum had some cards done for the funeral. They were like a memorial tribute, with a bit of a poem on them. Do you know the sort of thing I mean?’