Can someone please help me and my plumber who also appears flummixed by my problem My ocld water storage tank is overflowing - OK you say - fix the overflow Done it Problem seems to be that the water is feeding back in to the tank Plumber says it not my coil in the cylinder and I have new mixer taps so not them Any ideas out there?

Please explain your set up. Combi boiler?
coil in the cylinder Does that translate back into US talk as as a element in a water heater? New mixer taps on what?
Just off the top of my head, and without any hard knowledge of what you have, the first place I would check is the pipe that's feeding water back into the tank. Does it have a check valve? Is this the cold water supply that's running the tank over. Does this tank supply water to more then one unit?
Help me out here! I'm flying blind! Regards, Tom

Sorry Tom I will try to be clearer There are two gravity feeds coming off cold water storage tank One feeds two indirect cylinders which are joined with inch pipe work Both cylinders are in tandem This feed has warm water coming back up to the cold water storage tank If I close the gate valve then water comes out of one of the two vents to the tank The coils appear fine as no signs of water level increasing in lower header tank to heating There is only one mixer tap which is not passing cold water to hot side and showers are electric showers Is this of any help?

Let's work on our terms. When water's entering a storage tank through a supply it's called a "feed" This pipe fills the tank.
When water leaves the storage tank it leaves through a drain or discharge line. It may feed another tank but in the storage tank it's a drain or discharge line.
A vent is a pipe to the air that has no water in it. They provide air so air lock doesn't occur. Is the vent pipe the pipe water backs up out of? Sorry to be so picky about terms but mistakes are made when we're not on the same page. Would you care to repost? Thanks, Tom