I think you will find that all diesel cars take longer to start in the winter. Mine certainly does, the oil gets much thicker, also the glow plugs take a little longer to heat up,try charging the battery on a slow charge overnight, check in a few days tIme to see if it has lost any of the charge. Last resort go to local garage and ask them to check it for you they can tell if the battery is on the way out, I.e. not holding charge, dead cells in the battery etc etc , worth checking out before paying out.