Mystery water loss

My MY2000 MGF has been losing a bit of coolant for the last 18 months.... nothing serious just half a cupful every few weeks.



Last year I replaced the expansion cap, timing belt, tensioner, inlet manifold gasket, & as a precaution head gasket (Payen elastomer kit) everything was in A1 condition, as is the rest of the car. it's only done 42K miles from new.



I've now concluded that I only lose coolant if the heater is set to hot.. I also notice a hot antifreeze smell sometimes in the cabin.



I've had the heater set to cold now for a couple of weeks and the level hasn't moved.



Is this most likely the heater matrix or the valve assembly ??



Nothing is leaking into the car that I can see, or leaking anywhere outside.



Thx





Mat

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