Thanks so much for taking the time to respond - 2L - quite high mileage at 143k I brought it a couple of years ago and have it serviced twice a year, previous issues with dpf filter/turbo/glow plug modules but no issues at all for at least a year.
first time it didn't start I got towed to my local garage who did a diagnostic, a long list of fault codes appeared but he couldn't identify which codes were causing the issue even after clearing them all down and trying to see what came back - It started every time he tried it so I drove it away and booked into a BMW specialist garage a week later. before the appointment date it failed to start again (exactly the same) so I got it towed in to garage number two early. Once again it started every time and they couldn't find the root of the problem- as a double check they did a battery and alternator test (which were fine) and recommended I take it to the main dealer
test is £96 so not too horrific but I had a call from them this afternoon to say they were struggling to find the fault that is causing the non start - I hope to hear better news tomorrow but am at a loss what to do if the "experts" can't find it as like you say I could be throwing money at the symptoms...
Thanks again for responding - I'm learning more about cars than I ever thought I'd need to!