‘Yes. The first ones arranging the meeting, then when we were at the country park in the morning.’

‘When we interrogate mobile phone records, we get more information than just the number dialled or the duration of the call. We can also establish which mobile mast your phone was ‘talking to’ at any point during that call. Which means we can work out with a high degree of accuracy where you were – or at least where your phone was – when a call was made or a text was sent.’

‘I was at home on Sunday evening and at Fryent Country Park on Monday morning. So that’s what my phone records will tell you. Or they should do.’

‘Here’s the thing, Joe.’ He turned another page in his file, ran his finger down a column of figures. ‘When you sent those texts on Sunday, the records show they went via a mobile phone mast with the location designator A180Y2992073. Which is a mast in the steeple of St Michael’s Church, at the end of your road. It’s the nearest transmitter to your house.’

‘Well . . . that’s right, isn’t it?’

‘The texts sent from Ben Delaney’s phone also went via mast A180Y2992073.’

I looked from one detective to the other.

‘I’m confused. What does that mean?’

Naylor frowned, as if he was having to explain something very simple to someone very slow.

‘Rachel, can you elaborate for Mr Lynch?’

DS Redford took up the explanation.

‘Twenty years ago,’ she said, ‘each phone mast covered quite a large geographical area because there weren’t many of them. Since then, more and more masts have been put up to cope with the increase in mobile phone traffic. Now there are tens of thousands of them, they’re everywhere, and hence each one will cover quite a small area – particularly in cities with lots of people using their mobiles.’

‘Right, right. I get that.’

‘In this instance, both phones – yours and his – were within the narrowly defined range of the same mobile phone mast when those texts were sent and received. It means both phones were in very close proximity to each other.’

‘How close?’ Larssen said.

‘Very close,’ Redford replied.

Naylor said: ‘As if the same person was holding both phones, for example.’

‘Or if he was right outside my house! What the hell?’

Redford held a hand up, cutting me off.

‘I’m not finished,’ she said, her soft northern vowels a counterpoint to the hard knot of fear growing in my stomach. ‘On Monday morning we get the exact same thing happening, the same pattern, in a different location. Which is: two texts from you to him, two texts from him to you in reply, apparently to confirm that he was in the park and that he was alone. And again, both handsets within the very narrow range of the same mobile mast.’