Thanks for your replies...
Sorry I didn't describe the system very well.. My terminology may be highly suspect. It does say Heatrae Sadia (or similar) on the tank.
I will try the closing the mains tap and running the hot water to see. I suspect though the tank will empty first.
Im...
Hiya, what started as a simple job seems to have become complicated. My friend lives in a small flat and has a pressurised cold water system that feeds a wall mounted boiler. I believe it is a "megflow" system? I intend to replace all her taps, 2 basin and bath taps in the bathroom and 2 taps...