OK team, it looks like the heater control valve: temp' control to halfway, engine running, no "feel" of a solenoid operating.
I connected my circuit tester to the electrical connector and it flashed, so the control circuit is working.
Left hand hose (presumably the input from the engine) gets hot, right hand one (presumably the output back to the engine) is just lukewarm.
FYI, it is not on the back bulkhead, but inside it, under the "scuttle" panels - time consuming, but not impossible to get at. Also at least 4 jubilee clips, but that is not a problem until I come to change it - probably next Saturday.
Now all I have to do is ring my local Ford dealer and find out how much one costs. However, it will be a lot less than having them fit it as well.
As it is quite high up, I presume that, whilst some coolant loss is inevitable, I shouldn't find myself with a biblical flood?
Steve.