‘I’ve read widely,’ he said – suddenly cagey, which made me immediately curious.

‘What’s your favourite period?’ I asked.

‘What’s yours?’ he said, dodging the question.

‘I like the Romans,’ I said.

‘But you’re a policeman,’ said Martin Chorley. ‘Of course you’d like your brutality systemic and carefully licenced.’

Actually, I thought, it was the underfloor heating and the regular baths.

‘I like the Dark Ages,’ said Martin Chorley rolling the syllables around in his mouth. ‘When a man could make himself a myth.’

I could have talked archaeology and Victorian romanticism all day, but alas work had to take precedence. So while I let Martin Chorley monologue away, I laid my plans against him.

Anyone can sense another person doing magic, if they’re close enough and they know what to look out for. In fact you can’t learn magic without someone to demonstrate the formae first. Right from the start I’d wondered whether some forms were ‘louder’ than others and it’s not a hard experiment to set up. For once Nightingale didn’t object, partly because sensing formae is the key to winning a magical duel. But mostly, I think, because it forced me to practise producing a consistent effect, which he is very big on.

So we discovered that you can sense loud splashy spells such as impello or a fireball from as much as ten metres away. It’s down to two to three for normal were-lights and things like raising a shield, but less than a metre for certain variations on lux – particularly those that pushed the wavelength into the infrared. So while Martin Chorley indulged his strange need to confide in me, I slowly and carefully created a little invisible heat sphere, which I’m really going to have to come up with a name for, and nudged it in the direction of the nearest sprinkler head.

It was a top-of-the-line system and the reaction was almost instantaneous.

A good sprinkler system is gravity fed. The water comes from a big tank mounted as high as is practical and when the valves on the sprinkler heads activate, down that water comes. It’s a robust system with a minimum of moving parts and no pumps to malfunction at the wrong moment. The water keeps coming until the reservoir is exhausted.

I knew that and, judging by the peeved expression on Martin Chorley’s face, so did he.

I’d love to say I had a plan for what followed, but I’d be lying.

I used the distraction to ease myself into a slightly better position, palms down on the decking ready to lever myself over and up, but Martin Chorley wasn’t that distracted.

‘Oh no you don’t,’ he said. ‘Face down, hands on your head.’