The heater on my EBII Ghia has ceased to work. The climate control unit seems to not want to send a signal so that the hot water valve operates. Any suggestions?

First check that the vac tube is connected to the heater tap and that the valve works. Use a small hose and suck to see if the tap changes position. If all is well there then it could be the solenoid controller on the heater box or any number of things from there on.
I think there is a bit on problem solving in the climate control diagnostics or climate control fitting tech doco.

Is the heater valve normally in the open position? I can actuate the valve by applying a sucker to the pipe. Maybe the heater core is partially blocked??

If you can get the heater tap to work by sucking, put the proper pipe back on and start the car .note the position of the tap, then tell the climate control to turn on the heater. If the tap changes position then there is a blockage some where. It has also been known for the flap in the heater tap to become disconnected from the operating leaver so that no water can pass. You can also see how hot each heater pipe is when the engine is up to temp. I one is cool compared then the heater core may be blocked.
Another thing is it may have an air lock, this happens after you have changed the coolant or had a bad leak. Fill the coolant bottle up to the full mark and run the engine at about 2000rpm for 5 mins with the cap of the bottle and if there was an air lock it should clear it. Otherwise take off one of the heater pipes at the firewall and keep filling the bottle to force water through the heater, this will also tell you if the heater core is blocked.
See how you go, I hope this helps