Well, I took out the glovebox and it's mounting, and removed a cover under the dash, and I can now see the heater valve. It is leaking, so at present I can't drive the car too far without needing to:
1) Mop out the passenger footwell
2) Pour several bottles of water into the radiator
So, until I can get a new or repair the existing heater valve, I think that I will connect a hose straight across between the two places on the engine where the water hoses connect to the heater unit.
That way, I won't have a leak (once I drain the heater radiator!) and I won't have a hot interior, although I won't have a heater. But its summer so that don't matter for now.
Then I just have to find a new valve - any ideas? Was the same part used on other cars, does anyone know?