Well, as usual the answer was staring me in the face !!
Quite simply i was getting a hydraulic lock effect within the pump and clack valve.
all i did to over come the problem was to enlarge the water intake clack valve and replace stainless ball with nitrile rubber ones, these lift and seat better than S/S balls.
This allowed water to enter ram pump quicker= cylinder fills faster and empties faster with less resistance.
meaning i could use a higher speed geared motor to drive the pump as there is less torque required now.
All i need to do is perfect my automated water level/gas press/temp/boiler control system and it's sorted.
Cheers.