I reached for my phone and was about to switch it on and call for backup when Zach tapped me on the leg and pointed up the alley. ‘Look out,’ he said.

A figure came running towards us and I headed to block them.

‘Get back,’ I yelled and the figure reached for a gun.

There’s something unmistakable about the way someone reaches for a concealed weapon. The smooth way he pulled back his jacket with one hand while the other hand dipped under his armpit for the butt of the firearm. I didn’t give him a chance to finish the movement – I made the formae in my head, flung out my left hand, still holding my phone, and shouted, much louder than I’m supposed to, ‘Impello palma.’

Nightingale can put a fireball through ten centimetres of steel armour and I can singe my way through a paper target nine times out of ten but really, in the interests of community policing, it’s better to have something a bit less lethal in your armoury. I’d used impello in anger twice and managed to seriously injure one suspect and kill the second. More importantly, from Nightingale’s point of view, I was twisting the forma out of shape by making a sort of second off-the-cuff forma and ramming it into the back of the first. Turpis vox, he called it, the unseen word, and it was a classic apprentice’s error.

‘You think you’re being innovative,’ Nightingale had said. ‘But what you’re doing is distorting the forma. If you get into the habit of doing that then those formae won’t integrate properly when you start combining them with other formae to create proper spells.’

I made the mistake of saying that the couple of spells I could do seemed to work well enough, which made him sigh and say, ‘Peter you’re still learning first- and second-order spells. Spells that are designed to be easy and forgiving and that’s why you learn them first. Once you start getting to the higher-order spells there’s no margin for error – if you haven’t mastered the formae they’ll go wrong or backfire in unpredictable ways.’

‘You’ve never shown me a high-order spell,’ I said.

‘Really?’ said Nightingale. ‘We must rectify that.’ He took a deep breath and then, with a curiously theatrical wave of his hand, he spoke a long spell that was at least eight formae long.

I pointed out that nothing had happened, which prompted Nightingale to give me one of his rare smiles.

‘Look up,’ he said.

I did and found that above my head a small cloud, about as wide as a tea tray, was gathering. It looked like a compact mass of thick steam and once it finished growing, raindrops began to fall on my upturned face.