Strange problem

[BMWE34FAN]

Hey all, my diesel 75 has recently developed a fault where randomly it will take around 2 minutes to start but only does it at random times and there is no pattern of when it decides to stress me! It's not a starting issue like when you have a dead battery, it sounds like it wants to start. When it's running it's great (a tad blue smoke on start up first thing in morning for about 2 seconds) but does smell a bit rich (put it down to being a high mileage old diesel).
Any ideas what could be on the way out?

Many Thanks