An inspection of the black coil doesn't show any cracks, but there is a raised area that looks like a crack--however, I don't think it's a crack, I think it's a casting / moulding artifact. A close inspection reveals it's raised, smooth with no crack I can see. And the way it goes around the edge of the cylinder--I'd expect a crack to continue, this does not. The coil has been replaced, I think? This is the later BMW 12 13 1 244 426 / Bosch 0 221 500 203.
However! Close inspection of the coil led me to look at the (brand new) silicone plug leads, and...there's an area where the insulation has worn away! I'm thinking maybe under certain circumstances this could be shorting directly to the engine top cover where the plug leads lie, and so could be the culprit? I'll cover it with some silicone self-fusing tape (great stuff by the way, highly recommended) and see if the problem recurs. If it does I'll try to source a new coil.
The "crack" you can see in the photo looks for all the world like a casting / moulding artifact to me, but do let me know if I'm way off base here--I've left my magnifying glass at home.