I have had a series of problems with my X5
1) The first cost £1000 and involved replacing 2 injectors.
They were leaking and hence loosing common rail pressure.

2) Intermitant spinning but will not start.
Nice man from The AA attached a battery booster and INSTANT fire.
If the battery is >4 years old chances are under starting conditions the voltage drop is too great and the UCU thinks ther is a malfuction and does not trigger the injectors.
I replaced the battery and no more problem untill...

3) Intermitant spinning but no start!!
This time the nice man from the AA checked the OIL level (I new this to be neen the low mark but with a service due within a week or two!!) his comment was he had found on another X5 that low oil level was trigering the oil senser in the sump and again the ECU will not allow the engin to start. I put 1Ltr oil in and no problem for the past week.

Hope my rambelings help some one.