UK Car Repair: Vectra overheating, blown head gasket, header tank

Question
I have an 2002 vectra, it started losing a small amount of coolant every day ( but now has stopped? ) and  now it is overheating,I have wired the fan to run all the time but it makes no difference,if i run the engine with the header tank cap off coolant overflows out the top of the tank?
Any ideas.

Answer
How long does it take to get hot and overflow? A stuck closed thermostat could cause this so remove it for testing. If not, it is likely to be a blown head gasket.  There is a kit available to test the coolant for chemicals, check a tool supplier or have it tested by a mechanic. You could buy some stop leak and try it, but it may be too late.