My Transit will not start in the mornings?

When i goto start my van in the morning i just get a almost flat battery sound,like i have left a light on or something-but after the van has got going and warmed up it's fine.In the summer mornings it's much better,sometimes it starts like it has a new battery in it.
Most cold evenings i need to park the van on a hill,i just cant risk not geting the bugger started,i live in Sheffield so that's not too much of a problem.I have been told it could be something to do with the starter motor???