While I was messing with the black VW ECU (Sam working on his lap top) and checking thermo fan turn on temperature etc, I took a couple of photo's of the inner metal panels.
Quite pleasant in there! And apart from the replacement headliner, all original items.
After calibrating the TPS a few times, warming the engine up, messing with fuel maps, and generally being poisoned by fuel vapors, we concluded the TPS was failing after a while.
It seems to work until the car is warm and then goes full spastic.
So armed with this knowledge, a pair of new ones (1 plus a spare) have been ordered from https://www.cbperformance.com/ and will be here as soon as postage allows.
This is hopefully the gremlin I have been chasing for a while now.