Cooper looked at Tailby, but the DCI just nodded. He was a father himself. Cooper could only imagine how difficult it was to stand by and watch your child walk away from you into danger, when all your instincts were urging you to keep them by you and protect them. How much stronger must the feeling have been in Maggie, who had only just discovered it? She had finally found her child, only to face the prospect of losing her again. There was no way that she could have stood by and watched Ros walk off alone.

‘Yes, I drove her up to Ringham,’ said Maggie. ‘We were both so angry that we didn’t speak a word to each other in the car. I had driven right through Matlock before I even remembered to put the headlights on. When we got to Ringham, we parked above the village and walked up to the tower. Ros told me it was the meeting point, where I had to wait until she came back from the farm. I didn’t want to just sit and wait. Waiting was the worst thing. On the other hand . . .’

‘Yes?’

‘Well, there are other instincts, too.’

‘You couldn’t bring yourself to be too closely involved in a criminal act?’ said Tailby. ‘Is that right?’

‘All my training was against it, you see. All my beliefs. How could I? I was taking such a risk already.’ Maggie’s face betrayed a moment of appeal, a deep uncertainty. ‘Do you understand?’

Cooper looked away from the appeal. That was the crucial issue, in the end. Maggie Crew had waited at the Hammond Tower, torn between fear for the safety of her daughter and her horror at what Ros was doing out there on the dark moor. She hadn’t wanted to be there, but she couldn’t leave. Two powerful instincts had been battling inside her, and no doubt she had paced backwards and forwards at the foot of the tower, staring helplessly into the darkness, smoking cigarette after cigarette. Cooper could picture the unfinished butts tossed away, still glowing as they flew into the night. How many might Maggie have smoked during that time? Many of the cigarette ends would have landed among the trees on the slope, in the deep undergrowth. But not all. Some of them had landed on the ledge below the tower.

‘And what exactly was it Ros intended to do? Did she tell you?’

‘Oh, she didn’t just intend to do it. She actually did it,’ said Maggie. ‘She prepared her plan beforehand. She had two home-made petrol bombs hidden behind some loose stones in the wall of the tower. She had even collected some bits of rubbish and shoved them into the hole – empty drinks cans, chocolate wrappers, you know the sort of thing. She said nobody would bother to look in the hole when they saw the rubbish. I suppose she was right.’