Isolation valve keeps de-isolating

[RossssCan]

Really hoping someone who knows about plumbing can help

Had a plumber out today to look at a leak from a pipe in the bathroom.

He found the leak was from a failed isolator valve so he's fitted a new one. The pipe its self is cut off, I believe it was connected to an old hot water tank which is still there but obsolete. Instead of capping the water supply to this pipe, he's just put a new isolater on (he says as we're planning on putting a new bathroom in in the future, best to leave the supply there)

Plumber says the valve was 22mm (i might have misremembered that part) but he's fitted a 15mm.

Anyway, about 20 min after he left, the top of the valve has filled up and started spurting water, which we isolated. Called plumber back out who said its a new valve, there cant be anything wrong with it, and as we'd isolated it obviously we can't show him its still happening. Put it down to the new valve not being tightened enough.

Its now happened twice more. The valve seems to have opened slightly on its own, and waters coming out a little.
I can't find anything that triggers it. It's happened once when I've switched the boiler back on, but the other times the boilers been off so might be a coincidence.

Dont really want to pay for another plumber, as we can't show them whats wrong.

To top it off the main stop tap has seized (old house and recently bought it) so we can't switch the water supply off completely if going out/to bed etc, and really concerned as it seems to be happening randomly.

Really hoping someone can help. Sorry for the length.

you can get these valves for a pound a dozen, or for four pounds each.

the cheap ones leak, sometimes if you use them too much, sometimes if you don't use them, sometimes for no reason whatsoever.

don't buy this. No makers name.

buy this It is a good brand.

22mm is easily available.

If you will not want to use that pipe again, fit a stop end

That plumber is not following good practice, an isolator is only for temporary isolation. He should have capped it off with a proper termination.
Regarding the actual isolator it must have some rubber seal that's squeezed and popping open or something. As piglet John says it's basically too cheap and not properly made.
Ask the plumber to return and cap it off properly with an end cap and a pipe stub.

PS stop ends are suitable even if you will use the pipe again, you can even get push fit ones that can pull off without the need for tools.