In case this helps anyone - I was able to trace the leak and make the repair in the end. Steps were this:
Locate leak as closely as possible by isolating areas of the heating system as suggested below. Fortunately I already had to isolation values that separate the upstairs from the downstairs. With the valves closed, I confirmed that the pressure leak was still present, confirming the leak was downstairs.
I next bought a thermal camera attachment for my phone and a cheap borescope. The thermal camera was useless for this task. It was not the x-ray vision I’d hoped for. As soon as the source of heat is any distance behind another object, the heat dissipates quickly. I could see the warmth from the heating pipes under the floor, but there’s no way it was going to make a leak this size obvious. Maybe where the leak is closer to the surface (like underfloor heating) this could be useful.
Next I tried the borescope. This was a hit. I was wary of drilling holes blindly into the floor, fortunately there was some space by some down pipes going under a suspended wooden floor. Even more fortunately, many of the heating pipe connections were close to these down pipes. Even more fortunate than that, I was able to move this non-articulated borescope around in such a way that I got a visual on the leak.
Next I used some old photos of when I was laying the floor along with the pictures I got from the borescope to workout what part of the floor the leaking connection was under (counting joists etc).
With a good idea of where the joint was I used a hammer to tap and locate the joists under the floor.
I then cut a hatch out with a circular saw (having drilled a pilot hole to ascertain cut depth), and used an oscillating multi-tool to help get nice(ish) corners to the cut. It wasn’t as clean a job as I hoped though and the piece of floor I pulled out had to be thrown away.
The plumbing fix was easy, just replaced a plastic push-fit joint that had not apparently been done up properly.
To complete the hatch, I used some sheets of ply to make up the depth and some of the engineered oak boards I had from the original job, and got some trim for the inner and outer edging.
I have an issue where the pressure on my boiler is dropping to zero after re-pressurising over a period of about 15 minutes. The Heating Engineer(s) that have visited under a home emergency insurance scheme that I have have tried replacing pressure release valve (PRV) and have concluded that the leak must be in the pipes elsewhere in the heating system (which is not their problem as the policy only covers the boiler it's self).
There is no obvious escape of water, which makes me think that the source may be on my ground floor beneath a concrete floor in my kitchen or underneath a glued engineered oak floor in the other two downstairs rooms. Also, after shutting off isolation valves that isolate the bottom and second floor heating pipes, the pressure continues to drop.
I have used a thermal camera with the heating running to try and find the source of the leak, but as soon as the pipework goes below the floor, all that can be seen is vague blurred areas of warmth where the pipes follow under the floor.
I think I can hear water running, when I put my ear close to a vent in the glued wooden floor near some pipework. It sounds like water running not in a pipe, so the issue could well be there but it's impossible to be sure. The system is getting a lot of air in it, presumably because of the leak which is causing some extra noise.
I have tried using an endoscope around the vent area of the floor but there is no clear path to give me a good look around under there.
What are my options for determining the source of this leak? Is there a way I can avoid pulling up parts of my floors or using a specialist leak detection team?