This weekends DIY project involved wondering why the cistern is leaking into the bowl and the watertank pump keeps popping on every 30 minutes
So watch a youtube that explains that the valve seal probably needs replaced
Pull cistern apart to find valve seal that is probably as old as the house and completely crumbling so try to pull the old crumbly seal off and watch it crumble more and leave half of itself embedded inside the assembly so then dig the straggles out with a screwdriver and a stanley knife
Remember that the downstairs toilet is almost new and probably has a good seal (if the sizes match) and since the downstairs toilet never gets used its seal can be cannibalised until a replacement is sourced
Spanky salvage seal in a fetching orange hue
Different model cistern but looks like the diameters match
Slip it on to find it doesn't look quite right but give it try anyway and find it is still leaking.
Then pull the seal off again and apply slippery squeezy liquid to aid with the seal slipping into its seat
All's quiet on the waterfront pump again, jobs a good'un and my late great plumber grandad is smiling down from his pearly pew at this chip off the old block