Loft tank ball cock

Hi
The ball cock in my loft water tank is not shutting off properly when full. It still trickles out the overflow after sitting for a while, like overnight. I am going to replace it, & was just wondering if theyre all 1/2” connection as thats all I can find. Not sure what size mine actually is. Also, for the sake of £2 is it worth getting a new ball also? Should I replace it like for like (part 1) or do I need to go for part 2? What is the difference? Ive heard it could lower the water level, & thats not really ideal? Lastly, do I need any washers or will they all be supplied, & all I may need is a brass one for the mains connection?

Buy a full new valve and ball, brass arm type ,you need a new washer to connect to the pipwork

I can't really see why you wouldn't just replace the washer/diaphragm for less than a pound (just try to fit it the same way round as the one that comes out).

Considering it's a simple bit of rubber and how many times they flex in and out I think they're amazing things that last for years.


Change the whole thing if you want to but brass is brass and unlikely to fall apart and a plastic ball is a plastic ball .... if it's working and isn't punctured or degraded you're not going to see an improvement

EDIT.

I didn't read your post well enough. If the one you have is 'part one' then I can see you might want to swap to a new (part two) one for reasons of "upgrading". In which case my thoughts would be you'll only, possibly, want to replace the fibre washer when you fit the new ball valve to the old pipework.
We're assuming there's a handily placed isolating valve (which works properly) in the pipework to make life much easier for you.

If you’re replacing you would need to fit a part 2. The difference is that a part 1 no longer conforms to water regulations, part 2 has an air gap. Part 1 can be fitted with a double check valve on the incoming feed. You could try a washer as Mr.B has suggested, but I think you will need these:https://www.toolstation.com/ballvalve-washer-rubber/p31694

If the current Ballvalve has been there since around 1975, I'd replace the entire thing with a Part 2 version.

Cost of the new valve is easily offset by the avoidance of much swearing, possible loss of skin from your knuckles, the need to remove the old valve anyway and jam it in a vice, (before shredding the brass with a pair of Stillsons as nothing else is going to grip the valve body sufficiently to actually undo the blasted thing), then once you've finally managed to replace the washer, clean the internals out, and got it back in the cistern, only to find it leaks somewhere and/or you've forgotten to replace the fibre washer on the inlet pipework...