So I thought it was the batteries - got home last night, charged them both to full. Started this morning fine, drove about 7 miles, started again fine after that stop. About 5 miles down the road a light flickered on an off a couple of times, I think it was the glow plug coil, but it only flashed briefly. Another mile or two down the a1 and the dash lit up - Airbag, handbrake, VSC TRC, VSC OFF and battery. My dash cam also was off charge and running on battery and the cruise control stopped working.
Managed to get to my garage and it wouldn't turn on again so it has been left there and I'll wait for the verdict....