On Aug 11, 7:26*am, harry wrote:
The gate valves are usually about 4 or 5 turns fully open to fully
shut. *It's quite likely they are corroded up and can't be fully
opened/closed. Also the valve seats are quite likely corroded too &
the valve won't shut off tight.

Many thanks for all your replies guys. After reading that I had
another fiddle with the gate valves and managed to get the one
controlling the cold water feed to the cylinder all the way open. All
hot taps instantly fine again. Makes sense because they ran fine for a
few moments this morning before spluttering again which I guess is due
to the cylinder having a chance to fill full again overnight.

Looks like I'll have to look at replacing them at some point. Thanks
again.