Some times you can get the Doughnut to plump back up by putting in to soak in Hot water.
Len.
Good tip but wasnt going to happen with the original foam doughnut. It had welded itself to the old metal plate and parts of it came away when the cistern and pan were separated.
Cistern is now back on, properly screwed to the wall and no leaks. Only got the other two toilets in the house to do as and when they go faulty but at least I know now what Im doing (and they only have iso valves to mess with).
To be fair, the only reason I had issues were one, the original plumbing firm didnt properly fix the cistern to the wall imo and two, my last plumber did a good job with the fitting (it didnt leak) but I guess he didnt anticipate me having a go at replacing a faulty bottom fill valve.