My cistern inlet hole was seeping. I bought a new fill valve, removed the old fill valve, removed the sealant applied to cistern inlet hole, and installed the new fill valve.
What doesn't work
Tightening the plastic back nut (outside of cistern inlet hole) by hand doesn't compress the rubber washing seal (inside of cistern inlet hole) enough to stop the seeping. The seeping occurred at the same point as the old fill valve (outside the cistern inlet hole where the the back nut meets the cistern).
What does work
Overtightening the plastic back nut with spanner, i.e. 360° to 720° beyond what's recommended. I'm currently testing this using the back nut of the old fill valve. I'm also using a half inch plastic washer between the outside of the cistern inlet hole and the plastic back nut, like the old fill valve did. So far so good, but I'll wait a few more hours to confirm it absolutely prevents seepage.
The downside to this is that overtightening the plastic back nut on the brass shank of the fill valve could eventually overstress the nut to the extent that it cracks.
What might work
Using sealant. Will stop seepage and doesn't rely on risking overtightening plastic back nut.
Removing old sealant
I've gotten as much of the old sealant off as I can using a plastic card to avoid scraping cistern inlet hole. I think I need a sealant remover to remove what remains.
Cistern hole from above:
Cistern hole from below:
New fill valve:
Added plastic washer: