So the TPS parts arrived...and proved they were not at fault - hurrah - sigh... cue another week of looking for reasons which ended today.
The guy working on it - who is their chief engineer - has been in contact with BMW a lot about this and at one point they were telling him to replace the ECU. He didn't agree, and has steadfastly stuck to finding fault elsewhere.
They've pretty much stripped the loom and checked it for damage, but during test rides etc they'd noticed the QS was a bit...errr...sh1t. They decided to "re-code" it - they also took the wiring apart and renewed it. Initial impressions are; the QS is buttery smooth now and they have not been able to replicate the engine cut-out.
They are going to test for 24hrs...my fingers are crossed.
The great thing is at the end of all this that I've had the loom checked for all the types of failures people have reported before. I know I won't fall victim to them in the future. Silver linings...