An update:
All went fine, without a hitch.
I was proactively asked to stay on board during the BKK stop. I watched a nice film throughout (CX A350 IFE is gate-to-gate...watched it through landing, stop, and takeoff), whilst an army of cleaners and engineers did an incredibly impressive job in the 40 odd minutes we had on the ground with an 'empty' plane. (As an aside, the HKG-LHR A380 CW lower deck cabin was filthy, despite the many hours it was on the ground).
Bags also made it to LHR.
The only downside to the whole thing was that the shorter CX journey was quite a bit nicer than the BA leg. CX A350 hard product and service, even on short haul, I found to be considerably better than CW.
I will report back on the TPs. Expecting 40 but might get 80...
If anyone wants to do an AMS-BKK without even setting foot in BKK city or airport - this is the way to do it.