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I couldn't find a picture that looked like that one, hence the one that I posted as a 'similar' valve, for illustration purposes. Sometimes, if the float has sprung a leak, you can just tie them up for now, but even that wouldn't work in this case.I'm not familiar with that particular valve but it's easier to just replace the whole thing rather than try and fix it. Like gramaisc said get an isolated valve fitted below to make future maintenance easier.
I tend to use the fluidmaster fill valves. Easy to adjust and designed to be easy to fix
Sounds like you have split your diaphragm - oooerrrr.Here's a better picture, small amount of water comes out the top of that red bit too
Leaving it running overnight is only an issue if it's running your water meter up - it is worth being sure that you can shut the mains off easily though - it can get very exciting when a big leak happens...Yeah it does Luk like it might twist off but I dare not try it incase I break . I can only turn water off at mains , so if I break it I won't be able to turn water back on lol