I am trying to replace a tap but have turned the hot water feed pipe off at the valve
but it still comes out. Can someone please advise me how I can stop my hot water
(ideally without having to drain the whole immersion tank).

Thanks

dmkegs

With the valve closed open the hot water side of the fixture and then listen to the valve that feeds water to your fixture. If you hear noise the valve needs to be replaced. Go and try the main shut off valve and see if that stops the flow of water. There should also be a valve on the feed side of the hot water tank if you cannot locate your main shut off valve. Hope this helps

dmkegs

One other thing you could try if your not already done open & close the valve several times to clear any build up on the seat. Crud will build up in gate valves especially on the hot water side

labman

Unless there is an expansion tank inbetween, do not shut the feed valve off to the hot water tank or the main water feed without shutting off the tank.

With the water shut off, you can unscrew the insides of the valve and clean them. That may restore it. If you replace it, or any other valve, use a ball valve. Gate valves continue to be popular, but were obsoleted by low cost ball valves over 25 years ago. You would never know that to look at the selection in most plumbing sections. There is a current thread discussing this.