I smelt it, horse sweat and rough hair, and felt the weight and power of the muscles underneath its invisible skin. And then as if the moon had come out from behind a cloud I saw it, outlined in silver, as big as a carthorse, as shaggy as a pony and as pissed off as a bull in the household goods section of Marks and Spencer. Its mad black eye was fixed on me as it twisted and pulled, trying to get its narwhale horn free of the tree.

‘Peter,’ Beverley’s voice came from a surprisingly long way down the hill. ‘Don’t play with it – run!’

I know good advice when I hear it, and half scrambled and half slid on my bum down the slope, using the trees to keep myself from spilling over and breaking my neck. Above and behind me the unicorn snorted its frustration and stamped the ground. I was fairly certain it wasn’t going to attempt such a steep slope.

This is where the whole ape-descended thing reveals its worth, I thought madly. Sucks to be you, quadruped. Opposable thumbs – don’t leave home without them.

The trees ended suddenly and I joined Beverley and Dominic staring down a steep slope planted with white protective tubes and covered with nodding foxglove. I recognised it at once.

‘Pokehouse Wood,’ I said.

Had the girls been chased down here? Was that why Nicole had left a bloodied strip of her Capri pants on the barbed-wire fence – no handy fence-clearing magic for her. I wondered if there had been a moment when the unicorn had gone from invisible friend to terrifying predator – the point where the mask came off.

‘The river’s down there,’ said Beverley. ‘We need to get across it.’

What with the thigh-high grass, the nettles, the springy hummocks and inconveniently foot-sized hollows, it was harder work getting down through newly planted saplings than it had been amongst the full-grown trees. We were seriously grateful to reach the logging track that cut diagonally across the slope of the hill. At least, we were until my mental map of the area reminded me that further up the valley the logging track merged with the one in School Wood. A round trip of about a kilometre – or less than ten minutes as the pissed-off unicorn canters.

I pointed this out, and it was when we turned to flee down the track that we saw them ahead of us.

Two figures, child sized, white faces pale ovals in the moonlight, one of them in a green T-shirt, the other in a pink top,

‘Okay,’ said Beverley. ‘That’s strangely convenient.’

I heard hoof beats from up the track, two sets, in what I was to learn later was an aggressive canter – at the time it sounded like a gallop.

‘Not that convenient,’ I said.