roofrack said:I think JohnD has it sussed.
I reckon it's the mixer tap at fault and would prefer to fit a better one (or make the builder provide and fit one under warranty). I suspect it may be a ceramic disk-type tap, probably from Italy, and a UK approved one would be better. But I'm no expert.
You could fit non-return valves to make up for the bad tap, but this doesn't seem like the right solution to me.
If you find your main stop-cock, the cold water will stop. There may be a valve on the cold water supply to your cylinder that will turn off the hot supply (ths valve may be on a pipe coming out of the bottom of the cold-water tank in the loft); if not, once you've turned off the main stop-cock, you can drain the tank in the loft using the bath taps (bit wasteful).