I am assuming the hot and cold water supplies should be at similar
pressures. Is there a risk to the hot water cylinder if the cold is back
feeding to the tank--I note that those that have had a similar problem
in a gravity fed system have had header tanks overflowing! I have no way
of isolating the cold supply to the shower!

Any advice appreciated!

The plumber is coming back on Monday to look at the shower mixer.
The cold is not balanced properly rather than non return valves you need
a pressure reducing valve of the same pressure on the incoming main.
When I do unvented i always do that.

By doing this you will mitigate against future problems with mixer taps etc.

MORE NEWS