Starting problems for vectra B diesel?

I have a 2 litre Vectra TDI thats having problems with starting. Searching on the net seems unfruitful but lots of people have the same issue. The frustrating thing is people have looked at it for me and they can't fix it either!

Basically what happens is if its really cold like 1 degree or lower then the glow plugs kick in no problem and it starts within seconds.

However if the engine is a little warmer, say I've driven for 5 or 10 mins then left if for a couple of hours or its warmer outside say 10 degrees or so then it wont kick in straight away and takes forever, while trying to kick in there is a load of white smoke too but the glow plug light never comes on. Obviously this is taking alot out of the battery so now thats gone flat. :(

Ok so far i know that it's not the following:
- Glow plugs
- Glow plug relay
- Battery
- Alternator

It's like there is a temp sensor somewhere thats not telling the engine it's cold so kick in the glow plugs.

Any suggestions would be greatly appretiated!