I hesitate to speak too soon but.........
I have this afternoon changed the t'stat and HBV and put a new Cap on the header tank.
In the process I set to and removed the casting where the EGR valve sits. I did this because there was a small leak of coolant from this area and I guessed the gasket might be weeping?
On removal I found that some dork had actually fitted entirely the wrong gasket - hence there was a leak from the exhaust gas chamber as well as the water leak. I made a new gasket and liberally applied hylomar.
Re-assembly was the reverse etc etc - on the way I flushed the heater matrix by application of a hosepipe.
Result - not only does the heater work but also the lumpy tick-over which has been with me since I've had the car has gone!! It might be my imagination ( I drove for about 5 miles) but it seemed to pull much better too!
To say I'm pleased would be an understatement - I shall now see how long my home-made gasket lasts!

I guess the other conundrum is that I don't know which of the various components was actually the cause of the problem. However a good afternoons work and only £20 for the parts!!