Before you start spending big bucks check out the carpets either side of the heater matrix for damp.
My Cosworth was loosing water for a while and i couldn't trace where it was going,but one day whilst cleaning inside i noticed the carpet und underlay were sopping wet. I dipped my finger in the puddle and tasted the sweetness associated with anti freeze !!
Cheaper to replace the heater matrix but still a bitch of a job