2003 VW Polo 1.2 - Won't start when the engine is warm!

I have a very similar problem here. My car is as good as gold when the engine is cold, but if you drive it and warm the engine up, then try and start it again when the engine is hot (i.e. less than 2 hours later) it refuses to start.

I've talked to a garage briefly who think the temp. sensor may be the problem. I've also noticed it does get through a fair amount of coolant - it needs topping up every three months or so to stop it being below the minimum level.