How can I stop my Aquatronic 4 from leaking?

My Aquatronic 4 leaks from the bottom of the casing. It is NOT leaking from the PRD and I have changed all the o rings on the outlet pipe and have put a new PRD on but it has made no differance. The water that is leaking is cold, but the water coming out of the showerhead is hot. I have checked the main water pipe going into the unit and there is no obvious leak.
I have also replaced the solenoid valve assembly.
Any suggestions would be greatly received!