Plumbing – UK – Leak on inlet on toilet cistern – New Parts

Are you sure it's actually leaking from the bottom seal and not just flowing down from the top seal connected to the cistern? To be sure run your dry hand around the pipe near the top seal.

In any case to replace any of those seals:

1. Turn off the inlet tap to the cistern and flush until no more water remains in the cistern. (Have some rags ready anyway as there's bound to be a small amount of water coming out residual to the cistern.)
2. Undo the nut at the cistern end by hand and lower this end of the pipe--now you can slide it back towards the cistern to remove it from the bowl.
3. Replace any broken seals and reinstall (reverse of removal instructions).
4. Open the water to cistern inlet and flush several times to check for leaks. Make sure both the cistern and bowl are well secured as any possible movement left may cause leak during usage.

Thanks for the comments getterdun and bcworkz.

I went to Toolstation and bought a new service valve and also a new inlet valve for the princely sum of about £11. As per your suggestion I checked the plastic pipe, and it looked a bit rough around the end and one of the teeth of the thread was badly worn, but I couldn't see a way to replace just the pipe so I replaced the whole thing (dismantled the cistern and everything as per a video on YouTube).

I couldn't get the old service valve off, couldn't get enough grip to turn the valve part opposite to the nut part, so I decided to leave the old one now but give it a really good wipe and test anyway with the new inlet valve and pipe. I also used a fresh fibre washer. At first when I turned the water back on it leaked quite a bit and I was slightly disheartened, but I remembered reading somewhere that fibre washers slowly expand as they get wet, so I just put a bucket under the leak and left it for an hour or so. Bingo! No more leak :)