Aye isolation valves are all cheap shite these days they never last.
In regards to affecting the flat down stairs, I’d go chap the door and say I’m gonna do some work the water will be off for an hour they should be ok with that if not. Make up some excuse you had a burst pipe and had to switch it off to fix it 🤷🏻♂️ But realistically there’s something not right if the water stop cock shuts off water to both flats for the mains.
Aye isolation valves are all cheap shite these days they never last.
In regards to affecting the flat down stairs, I’d go chap the door and say I’m gonna do some work the water will be off for an hour they should be ok with that if not. Make up some excuse you had a burst pipe and had to switch it off to fix it 🤷🏻♂️ But realistically there’s something not right if the water stop cock shuts off water to both flats for the mains.