Hi Simon,
Is it sticking on the rubber seal around the door? or is it the door hinge or door handle that is a bit stiff?
The door rubbers are easily lubricated with some silcone lube spray, which can be bought from any car spare shop (incl Halfords!). If it is the hinge or the door handle I will defer to those who know better! But I would guess that it is just a case of lubricating the appropriate part with the appropriate grade of oil.
Good luck!
David