Hi Colin, I have started to see a few of these now with the cam timing jumped causing this problem.Once you have eliminated the cam and crank sensor signals as suggested by Alan I would then remove the rocker cover to check the timing .You will need lock up tools and a pin for the crank to set the timing . If it is in fact timed incorrectly I would reset it and replace the chain tensioner ( the few I have done have seemed very weak)and there is a modified retrofit chain slider to fit on the front cam Carrier to stop the chain jumping , available from BMW. If you need the part number or any further info let me know .
All The best
Steve