Right back with a bit of resolve for this.
I didn't as of yet check the compression as it didn't seem logical to me but please do say if I'm utterly wrong here. I'd imagine if the compression was down, power would be down. I've had compression issues before on my track car due to rings, so I've experienced it somewhat and yes, power was getting worse with all of the other tell tail signs.
Anyway so the short version is this.
I was sent a new TDI tuning box, fuel economy issue solved.
(actually slightly better as I'd done the other work)
If a problem occurs again (still has 4 years warranty), I keep a couple of appropriate tools in the car to whip off the bumper and unhook it. I've also made a dopey bit of protective covering as rain water goes exactly where the box wants to sit.
Anyway, the car has a multitude of other annoyances now, and I've lost the will to fix anything else on it at this point
