I need some detail of the system & layout;
Exactly what size H & C water storage tanks do you have & where are they located?
What type/make/size of pump do you have & where is it located?
What type of HW cylinder do you have?
What type of flange do you have on the HW cylinder?
What size is the feed pipe from the HWS tank to the pump?
What size is the feed pipe from the CWS tank to the pump?
Where is the CW feed located in the bottom of the tank relative to the ball valve?
What is the distance between the bottom of the CWS tank & the shower head?
Do you have any up & over pipe loops from the pump to the shower head?
If you do, what is the distance from the bottom of the CWS tank to the pipes?
If you up & over loops, are they fitted with vents?
Do you have any up & over loops in the supply pipe work from the tanks to the pump? If you do then you shouldn’t!
As stated a one way valve on the vent pipe is a bad idea & is not going to cure it, you have another basic problem somewhere. Essex flange is best IMO but lets see what else youve got gong on there; a sketch or even some photos would also help.
