In the darkness I could only make out the surrounding hills by the way they blotted out the stars, but according to the map on my tablet the shadow to the west was Pyon Wood and to the east Croft Ambrey, with a waning moon hung above like a banner. The Roman road was a straight grey strip between the black hedgerows. I parked the Asbo on the grass verge and left the hazard lights on. Beverley held the torch while I detached the detector from its mount and carried it back to the car. I cracked open the PVC case to expose the bare innards of the device.

‘That’s a mobile phone,’ said Beverley, leaning over my shoulder to look.

I explained that it was, and that the detector worked on the simple principle that a powerful enough source of magic would break the phone and cause it to stop pinging the network, which would then alert the custom program on my tablet.

‘So basically it only works once,’ she said.

I used a jeweller’s glass to scan the electronics, but I couldn’t see any visible pitting.

‘That’s the trouble with magic,’ I said. ‘It’s slippery stuff.’ I shrugged. ‘What are you going to do?’

‘You could bundle four or five phones together and automatically rotate through them,’ said Beverley, as I bagged and tagged the phone for shipping back to Dr Walid. ‘That would extend the life a bit.’

I installed one of my spare detectors by the stile and packed up.

‘But the switching mechanism can’t be a microprocessor,’ I said. ‘And I haven’t had time to test the effect of magic on transistors yet – you might have to use valves or electromechanical switches.’

‘Do you know why it happens?’ asked Beverley as we drove back to the cowshed.

I admitted that I did not have the faintest idea how magic did anything – let alone why it reduced microprocessors to sand and brains to Swiss cheese.

‘When you do magic . . .’ I said.

‘I don’t do magic,’ said Beverley quickly. ‘You get me? It’s not the same thing.’

‘When you do . . . things that other people can’t do . . .’ I said, ‘it doesn’t damage your phone.’

‘Not unless the waterproofing fails,’ she said.

‘I wonder why that is?’

‘That’s easy,’ said Beverley. ‘I am a natural phenomenon. So I do less damage than you.’

‘Have you visited Covent Garden recently?’ I asked. ‘They’ve almost finished the rebuilding.’

‘That was collateral,’ she said. ‘And entirely your fault.’

The next morning I decided to check out Pyon Wood Camp – I took Miss Natural Phenomenon along with me, so she could tell me what all the plants meant.

‘They mean,’ said Beverley when she saw them, ‘that in lowland Britain if you don’t chop the trees down you get a forest.’