An update: with the upstairs wall thermostat turned down to 16C and the UFH thus effectively switched off, the cracking noises seemed to have stopped altogether, but then there was an isolated couple of them (not as loud as the worst of before) quite late in the evening. I couldn't see or discover any specific triggering cause.
The temperature in the two rooms in the loft conversion dropped as expected, stabilising around 21C thanks to the heat from downstairs. This was just about tolerable for the bedroom but definitely not so for the bathroom, which unfortunately wasn't given a separate zone of its own.
This evening I returned the stat to 23C and it wasn't too long before the noises returned, almost exactly as before. I don't know if I'm conveying just how loud and startling they can be: I'd record them and post an audio file but without a reference level I'm not convinced it would mean anything.
So, the experiment established that it's the heating which is causing the problem; now I have to decide what to do about it.