I agree with the plumber the flow is probily short circuiting through the cylinder and back to the boiler it just needs a locksheild gate valve on the cylinder return 10min job
if you close one pump valve it helps just remember to turn it back on Also some tank connectors on the F&E tank have two little bits inside the fitting that stops the bung from sealing so it will not work
I have spoken to bengasman via message and i was wrong to think he was not registered and i would like to appologise to him and i hope we can put this suject to bed
many thanks Paul
Rules are rules if we all thought like that why would we pay our gas safe every year when we would only have to pay for our ACS every 5 years to remain competent YOU ARE A FOOL GOLDTURD