Sound like a HGF to anyone?
Car loses a litre of water every 15miles or so, also the header tank does have a funny whiff to it.
No other signs at all, no oil in anything, no water in anything, no big plumes of white smoke from the exhaust. I have checked everywhere for a leak, but can't see a thing.
Its 2wd with a 200 b/e, group A gasket that is only 2000miles old, only running stage 1.

 

Get a compression test done on the cooling system, could be a leaking from anywhere, rad, stat housing, pipe, anything.

 

and a gas test in the tank - they'll soon tell you if the HG has gone. Does sound very much like my saff a few years back though. The grp A gasket gave up in a very minor way but enough to lose water, a top up every 100 or so miles though so not as bad I guess.

 

Had a test done this evening, some handheld pump and some blue liquid.
Liquid turned a greeny yellow. Bye bye headgasket.