X5 starting woes

[nick francis]

My 4 years old X5 3.0i Diesel has not been happy in this cold snap. The battery seems to be failing to the extent the car wont start. Despite several long runs its still very laboured and on Sunday I had to get the AA to come and jump start the car. Aftre that I drove 70 miles home, in Transmission Failsafe Mode (car auto protecting itself I guess)
Any ideas as to the problem
1) duff battery
2) faulty alternator
3) Is the diesel warm enough to ignite?

help would be appreciated