Timing chain issue - my BMW 320D 57 plate engine.

Timing chain issue - my bmw 320d 57 plate engine has just had the timing chain snap, is this normal at 109000 miles, is this due to wear and tear? I have full bmw service history, I've been told that is could be due to timing chain tensioner failing, is this wear and tear, basically my warrenty company have rejected my repair on wear and tear grounds. What are main causes of timing chain snapping or timing chain tensioner failing, is this common at 109000 miles, how long should a chain and tensioner last on this engine. Mechanics said there is only slight wear and tear on all the components of the engine. Please help; I am struggling with a £5.5k repair bill which I cannot afford and just need more information or BMW literature on these parts and when they should be relaced or how long they are expected to last