If its clicking then it sounds like you have a flat battery. Batteries do not like the cold so will be less efficient, however a good batery in good charge will still be fine. So either you have a battery that is past its prime and needs replacing, or, the battery has not been given enough charge between usage.
If you have done lots of short trips, you may be taking more out of the battery than you have put back as it will take a while for the vehicle alternator to replace the charge taken from the battery to start the engine. (Sorry I have no idea how long I only know the theory lol) and starting a very cold engine will take more Ooomph out of the battery than starting a new one.
What to do?
You basically have 4 choices
Call the AA if you belong to a breakdown
Ask a neighbour to jump start your car if you or themm have jump leads
Buy a battery Charger and put it on your car and leave for several hours
Replace the battery.
Note, if you buy jump leads, make sure you buy heavy duty ones, do not be fooled by cheap thing ones, these are for lower amp pertrol cars and will not be adequate for a diesel that needs lots of amps to get going.
Steve