Noticed im loosing a litre of coolant a day over 20miles,

Water is being blown out of the overflow pipe,

Car drives as normal, no visible leaks or overheating,

No smoke on start up or when driving,

Suspecting the head gasket??

Hi a litre of coolant is a large amount to lose in such a short time. If as you say you have no overheating problem ,my first suspect would be the radiator pressure cap .Replace this with a new one ,if your leakage still persists get a pressure test carried out by a garage who should be able to verify whether the head gasket is faulty or not .

thanks true blue,
Ive had a new cap put on a few days ago when this problem occured. Apparently it has made it worse, the engine temp is now staying cold and it is always on fast idle. Thermo was changed a few months ago, but iam going to take it out and recheck it just incase the unit has gone faulty

pressure test would be the way forward in your case.

symptoms sound like the head gasket which is metal, but im sure more members on the forum can help you out.

This is a block tester kit, its used by mechanics to test for head gaskets, if compression is leaking into the cooling system it turns the blue fluid yellow. if the test comes out OK, you could have a circulation problem.

http://cgi.ebay.co.uk/BT500-TEST-TOOLS-BLOCK-TESTER-KIT-/310214882652?pt=UK_Hand_Tools_Equipment&hash=item483a3f755c