Frustrated by the fact that both the washer motors for (a) the windscreen/rear window and (b) the headlights seemed to run satisfactorily but no water emerged, I finally pulled the whole lot apart which annoyingly includes removing the whole wiper motor and drive train assembly. There are a few tricks here and I am happy to pass these on if you are doing this.
As getting (a) to work was the priority, I focussed on it and as it seemed to be spinning properly I carefully pried off its "valve" assembly and turned on the washer and water spat out. So far so good. The valve assembly, a black half and a white half, both of which had outlet nozzles, appeared designed to snap together, but are possibly glued, and did not want to come apart. I eventually separated them by progressively trimming the outer sleeve.
Inside I discovered a simple plastic plug, about 12mm long by 8mm dia. with rubber pillows on both ends which sealed the outlets. I guessed its job was to stop drain back to the washer bottle from the pipes to the bonnet nozzles and the tube through the roof to the rear window washer, thus keeping then primed. I guess that with the passing years what were probably once soft pillows and hardened and expanded, and the pressure from the washer motor could no longer compress them enough to let water pass.
I carefully ground a little off each pillow, reassembled the valve unit with a smear of silastic and voila, the washers worked, albeit grudgingly. A few weeks later, they stopped again, so I pulled it all apart again and removed the little plug completely, wow, water jets to the windscreen. I anticipated a delay in the washer response due to drain back, but so far that has not been apparent.
So far the washer to the rear window does not work so I need to explore that further. (b) is another matter as satisfactory closing of the valve assembly is essential to stop the draining of the washer bottle through siphoning, as the jets are lower than the bottle.