Another little update:-
I plugged the hose pipe in here:-

......and you should have seen the state of the fluid that came out here to start with

:-

......as you can see, now running clear. I also removed the thermostat housing assembly:-


.......and squirted a load of water into the head where the thermostat housing was bolted to:-

Its now running clear from everywhere so next job is to clean the mating surface, fit the new housings, thermostat, cap etc., and await delivery of the new pump, probably Wednesday, if I'm lucky Tuesday.