Might be teaching my grandmother here but....
I have had a problem with gate valves that don't quite shut off recently. They allow a dribble of water through which isn't good given the recent weather. Given a normal layout which involves a gate valve adjacent to the cold water tank which isolates the water supply.
As "ball valves" and "stop valves" restrict the bore (and as we are talking about pumped systems) I had discounted their use.
However I have found some "quarter turn, full flow, ball valves". They do not restrict the flow and are as guaranteed as anything that I know of to "shut off". Cost under a fiver from plumbers merchants for a 15mm, 22 mm also available. They also look as they will fit in the same space as the gate valve so should just need 2 nuts unscrewing and a quick replacement.
Don't forget to drain the tank before you try it though!!
Regards.