I was able to restore my car's corroded heater valve to good operating condition by soaking it for a couple of days in a household-type 'de-liming' liquid (same stuff that's used for cleaning up kettles and coffee-makers). The product I used is called 'C-L-R' (stands for, calcium-lime-rust, IIRC). Probably not available in the UK, but I'm sure you'll be able to find a similar product without much fuss. These heater valves are relatively bullet-proof... except for the little seal where the control rod exits the main valve body. If you take the valve out of the car to refurbish it, make sure you pressure-test before you refit it. If the little seal leaks, you're probably best to buy a complete replacement valve. To pressure test, cut a length of skinny bicycle inner tube, complete with the valve stem. Seal off one end (clamp it), then clamp the other end over a short length of heater hose. Then clamp the heater hose onto one of the heater valve's ports. Put your thumb over the other port, then pump some air into the inner tube. If you hear air escaping through the control rod's seal, it's time to order a new heater valve.