My X5 3.0d E53 has started fine on first turn of key for the 10 years I've owned it, then last week it took 2 turns of the key, then yesterday it took 3 and now this evening she won't start at all. Starter turns engine over fine, battery fully charged so not that. Car has half tank of diesel.

Any ideas where to start investigating? Searched on here and other internet sites but nothing the same found. Any ideas?