Start with the battery. Remember that it is only able to deliver about 70% of its rated CCA at 32 degrees fahrenheit, and 50% at 0 degrees fahrenheit. If the battery passes the load test, then the starter can be tested to make sure it is drawing enough amperage to spin the crankshaft fast enough. Next, I would check the glow plug controller. Are they being energized at the correct engine head temperature and are they staying on long enough?

Although my last diesel was a 1981 Pontiac Bonneville with the 5.7L Oldsmobile engine, I did manage to roll up 264,000 miles on it before selling it. That engine had a 22.5 to 1 compression ratio which gave it a very high temperature rise, but the glow plugs would still energize anytime the coolant temperature was below 100 degrees fahrenheit. They would remain on from 5 to 30 seconds depending on the actual temperature, until the "wait" light would go out. Of the five or six "no-start" incidents I remember, two were from gelled fuel, one was the glow plug controller, and the rest were from weak batteries.

2002 X5 3.0 243,000 miles
2004 325i 108,000 miles