stewartacraig

Thanks for your reply, AJS Heating, and for clarifying the point about the cold water feed...

There are two tanks in the loft which I may have confused... A little one which appears to be fine (perhaps this is what is called the F/E tank?). Then there is a much larger tank connected to the hot water cylinder (perhaps this is the CWS (cold water storage?) which is the one that keeps overflowing. The base of both tanks sit on the same wooden supports so they're both the same height.

I do have a mixer tap in the kitchen and one in the bathroom and a thermostatic mixer shower which are all cold mains and gravity hot so I guess I need to inspect each of these for a non-return valve issue. Not sure how I'll know if they're working correctly though... Any ideas how I can test this?