Thanks for the advice.
I've checked the filters and they are clean.
With the shower head removed the flow was :-
Power off - 8.2 litres/min
Power on - 12.8 Litres/min
Similar figures were measured with the shower head replaced and with a different shower head.
However, the flow...
I have replaced my gravity fed shower by a new shower cubicle and a Easiboost 2 shower pump. As I live in a bungalow the original gravity fed pressure was quite low and the intention was to boost the pressure and flow rate accodingly with the 2 bar pump.
Following the manufacturers...