Posted by David Jupp on 21/11/2022 08:55:40:
…
As the valve is inside (and assuming it to be a 1/4 turn) valve, it is unlikely to have seized – if it has that will become obvious when I attempt (carefully) to turn it.
Replacing the valve would require removing/replacing the boarding, and consequent re-decorating – hence I'd like to try closing the valve through the access hole that was clearly put there for that purpose. …
I don't think the valve being inside has any effect on whether it's seized or not. What matters is the nature of your water, hard soft etc, and – most of all – how long it's been since the valve was last moved.
When my interior stop-cock seized I was able to force it closed but the cock twisted on it's compression fittings, damaged the pipes, and both joints started to drip. I managed to fix that by retightening, but it left the pipes in a precarious state. About a year later, I had to turn off the stop cock again, and the same thing happened except it was impossible to stop the leaks. Fitting a new stop-cock to cleaned up pipes became a nightmare when it turned out the incoming plastic pipe needed an imperial top-hat gland that's unobtainium. (being a metric fanboy, I used 'industrial language' on the idiot who had decided in the early 1970s it was OK to make a modern pipe and seal system in imperial rather than going metric; it seems the imperial version didn't last long, making spare parts hard to find, turning a straightforward job into a pig.
I agree it's worth a try. However, I recommend preparing for the worst! Don't attempt it when the shops are closed, you don't have the right tools, and plumbers are on strike!
Dave