gidpor, thanks for your rapid reply. My car is automatic transmission. Oops, just realised what you mean. It's manual-controlled dials on the fascia for the heating, blue thru' to red. No AC. Had the valve-thing replaced two times: the whole unit, I think, kept the old ones as I'm sure they're working OK, and were changed needlessly, perhaps. Still, it all gets rid of money, doesn't it? I think it's a duo-valve as there are two clear outlets: like a fat Y-shape. Fits under the bonnet (hood) just in front of the windscreen (windshield), centre of the car. Mechanic proposes to disassemble control panel inside car to have a look, but this of course will cost money.

Grateful for your thoughts and any others.