It was the water level in the cold water supply tank.
Firstly let me thank Softus for the feedback on the SureStop water taps, I will heed your advice.
Thanks to everyone that has offered help, in particular TicklyT whose suggestion that it may be associated with the cold-water header tank has proved to be the key to my problem.
Update for any that may be interested.
When I finally looked at the cold-water tank the ball cock was not shutting off correctly and allowing the water to overfill by approx 3” (75.0 mm), over filling the tank to just below the overflow outlet. Crudely measuring the two levels this meant that the cold water level was about 25.0 mm above the level of the hot water overflow pipe. I replaced the cold-water ball cock, I then let the system run completely normally from Friday lunchtime until this morning, checking both tank levels twice a day, both tanks have remained at the correct set levels.
What a relief.
That said, both my wife and I are now noticing a difference with the performance of the shower, its not as forceful as it was. This has only happened since the above rectification of the cold-water ball cock, could the above tank levels have damaged the pump in any way? Additionally, when we rotate the showerhead to the massage position (Pulse mode) the pump cuts in and out continuously, almost in sync with the shower spray pulses. Note the pump ONLY supplies the one shower and NOTHING else.
A very grateful,