oh wrote:
Small plumbing snag: occasionally there's a bit of hot water coming out
of my cold water taps: I open a tap, the water runs cold for a few
seconds, then suddenly hot for another few seconds, and then cold again.
And if I turn off the water at the mains and open a cold water tap, the
water from my hot water tank eventually drains out through the cold
water tap. Shouldn't there be some kind of non-return valve ensuring
that the hot water doesn't drain back into the cold water side???

Called the various plumbers and the guy who installed the hot water
cylinder came out. What I thought was just a T to a pressure relief
valve just before the cylinder was indeed a non-return valve as well. Of
course it was stuck open, however it was only a five-minute job to
replace it once the plumber got going. Thanks all for your replies!