So, it's been back what, a week or so? Alarm problem was solved by unplugging the bonnet sensor, and removed as it was falling apart. Happy days... For a week!
Rewind back to 11th June, my sis took the motor up to Newcastle, the ASC light made an appearance so I got a call, ok'd her to carry on driving there and back. When she got back I checked the tyres and there was 1x mismatch on the rear and also the tread depths were different.
I drove it around on the 15th before I took it in for tyres and the light had disappeared so I'm assuming at high speeds the tread differences and possibly inflation pressures made enough of a difference to trigger the ASC light. I had it on my e36 too. So, I thought I'd try that option first.
Got to my tyre garage and they swapped the right rear to the pass side front so the fronts had same tread depth and then put a new Bridgestone on the rear to match the other one. Then tyre pressures were corrected, 32psi front, 37psi rear. £25 later drove out and the car instantly felt miles better.
Got home and parked the car up, went inside and carried on with my day.
Yesterday (16th) my sister tried to start the car to go to work, and nothing. No clicks, no starter motor struggle, nothing. Not a sausage. I feel to wash my hands of this motor now, I have spent almost £1950 in the two months we've had it, and I feel I could have bought a much newer car for this much money. I just want to set the bastard on fire now.
Enough is enough, never ever going to buy a bmw diesel again.