What a f**king, stupid, annoying... arrghhh!
So, I decided to do an oil and filter change yesterday. After discovering that one of the threads that the undertray bolts to was just spinning in the chassis leg and realising I'd have to prise that bit off (plus spotting that two of the bots holding the corners of the tray to the bumper had previously been sheared) I proceeded with the oil change and then went to do the air filter. Anyone who has done the job on a GT will know that it is the most idiotic of designs for a serviceable part in terms of access.
Anyhoo, after removing the front fusebox and popping various fixings out of their locations I had enough access to slide the new filter in, but a quick look around revealed that whoever had done the job previously had removed the two bolts connecting the outlet of the airbox to the turbo hard pipe and had only put one of them back. When I decided to fit a bolt there I realised why he/she had decided not to bother - after I had removed the battery, battery plate and most of the right hand side of the engine bay, along with the skin on both my handsl.
Re-assembling everything (7 hours later) and I called it a day.
Went to work this morning and first thing I was aware of was that my reverse-park sensors weren't working. Neither were my reversing lights.
Yep, that's right - I've since discovered that I've ripped the wires out of the reversing light switch on the top of the gearbox while removing the battery plate.
Only bright side is that it took me one search here to realise what the problem was.