‘Mm-hmm,’ I squeaked. He was obviously using this as a hunting ground for fresh targets, just as I feared.

As soon as he’d moved on, Leo and I dropped down to the ground again. My heart was racing with adrenalin.

‘We’ve got him. We’ve really got him,’ I said, hopping around on the spot, unable to keep still. ‘I can’t believe how unassuming and normal he looks, apart from the dodgy vest, of course. Should we do a citizen’s arrest? Is that even a thing? And if so, how do they work?’

Leo frowned. ‘It’s not a thing we’re going to be trying in these surroundings.’

‘But we can’t let him get away with it,’ I said.

‘I wasn’t suggesting that we do.’

‘Okay, so what now? What would you do if you were still in the police?’

‘I’d invite him in for questioning, but as we certainly don’t have enough evidence for an arrest, it would have to be a voluntary interview.’

I nodded. ‘That makes sense. As we can’t exactly bring him in for an interrogation without revealing our hand, I guess our next step is to find another way to get enough evidence for that arrest. Then we can take it to the real police. I mean, the ones who are still serving. Sorry, I didn’t mean to rub it in.’

He absent-mindedly put his arm around my shoulders and squeezed. ‘Don’t worry, you need to try harder than that to offend me. How do you feel about doing some surveillance? Now that we know what Mr so-called Brian James really looks like, we need to press home our advantage.’

‘I think I can see where you’re going with this,’ I said. ‘We follow him around the course, observe every interaction, and then tail him home so we can find out his address. Then I can go on to the electoral register at the library and see who the registered occupant of that address is, and then there we have it, the real identity of Brian James.’

Leo blinked. ‘Okay, I hadn’t thought we’d go quite as far as tailing him home. Perhaps that’s a tad extreme. But the observing every interaction at this event bit sounds good.’

I shook my head. ‘Where’s your sense of adventure? This is the biggest breakthrough we’ve had. This time tomorrow our whole investigation could be successfully completed. Or at least, well on its way to being,’ I added hastily, realising that I didn’t feel the delight I should at the idea of accomplishing our aim. Because that would mean I no longer had a legitimate excuse to spend so much time with Leo.

‘Let’s see how we get on,’ he said.

‘Okay,’ I agreed. ‘We need to start now, then. We’ve spent too long talking about this already. I hope the next obstacle is a tough one so he’s still stuck on it, otherwise we’re going to have a job tracking him down again.’

It turned out to be practically impossible. We had to negotiate our way across a wide stretch of icy water by hopping across a series of tiny platforms which wobbled wildly as soon as anyone landed on them. Most sensible people were giving up fairly early on and trying to swim instead, but our quarry seemed determined to do the thing properly. He was currently balancing on the middle platform, urging the woman with the dreadlocks to leap across and join him.

‘I’m a bit worried that she’s going to be his next target,’ I said to Leo as we pretended to be working out our strategy for getting across. ‘He certainly seems to be very interested in her.’

‘Then we need to get her away from him,’ he replied. ‘But to do that, we have to get across this water. Ladies first.’

His expression was gleeful.

‘Oh no, I think, in this instance, I will let you lead the way,’ I said. ‘If only one of us is moving at any one time, the other can be observing what Brian’s up to.’

I was pretty pleased with that excuse. In reality, I was wimping out at the thought of ending up in that muddy, cold lake. I knew the worst thing that would happen was that I’d get wet, but I couldn’t tell how deep it was and I’d never liked the idea of being in water where I couldn’t touch the bottom. Who knew what kinds of creatures might be lurking in the depths ready to grab hold of me? Not that I wanted Leo to know I was thinking that way.

He shrugged. ‘Time to learn from the master,’ he said, exuding confidence from every pore.

‘Show off,’ I retorted.

Leo winked. ‘Isn’t that exactly what this whole event is about? It’s the human equivalent of chimpanzee chest-beating. I’m merely getting into the spirit of the occasion. Here I go.’

And with that he leapt from the shore to the first platform. For a moment I thought he wasn’t going to make it, but he landed with effortless ease and then pulled himself up into a gymnastics-style salute to his imaginary audience.

Although I was actually impressed by his athleticism, I pointedly yawned in his direction which made his grin grow even wider. Looking beyond Leo, I clocked that Scammer Brian had moved onto the penultimate platform, but there was no sign of the woman with dreadlocks. I scanned the swimmers in the water and realised that she’d joined their number and was doing a slow breaststroke alongside a man who seemed enraptured by every word she was saying. At least that was one less thing to worry about.

‘Hurry up, Leo,’ I called. ‘Jump onto the next platform so I can make a start. There’s a queue forming, you know.’ What I actually meant was for him to get a move on, so we didn’t lose sight of Brian.

I blamed myself for what happened next. Recognising the urgency in my tone, Leo didn’t take the same amount of time preparing for his next leap as he had with his first one. He probably wasn’t helped by the fact that he was now setting off from an unsteady surface. Whatever it was, he jumped and caught the front of his foot underneath the next platform. I watched as he tumbled forward almost in slow motion, hitting his ribs on the edge then rolling off the platform with an almighty splash.

I didn’t even stop to think about it. I half-dived, half-leapt into the water and started a messy front crawl to where I’d seen him disappear under the surface. About two strokes in, the cold hit me, instantly turning my breathing into ragged rasps and numbing my brain, making me uncertain which direction I should be going in. I paused for a couple of seconds, forcing myself to take a deep breath and exhale slowly so I could get myself back under control, and then I resumed my desperate swim towards Leo.

‘Are you okay?’ I called towards him as his head broke through the surface. His expression was glassy and I recognised the effects of the cold shock that I’d just experienced.