The most probable cause is simply the fuel.
In most cold weather countries the winter grade diesel has a higher volatility rating and hence is easier to start. This fuel is not readily available in the UK winter. The 'summer' grade diesel is likely to turn a little gloopy during very cold weathe, ive seen vehicles with poor quality summer fuel fail to start in just -2degC weather.
Unfortunately the only fix is to perhaps call your local garage manager/supplier to see if they use summer or winter diesel through the colder months...
All the best