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.

As other posters have said, a hot pipe is close to the feed pipe for the
cold water tap.

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???

Hmmm. I would expect that that normally the hot tank would be fed by
a header tank above it. Typically in the loft.

Perhaps your hot water tanks cold feed is directly connected to the
incoming mains cold water. Thus, if you turn of the main stop cock
and open a cold tap the water in the hot tank could flow back out
of the feed to the cold tap.

Guy