I had similar, ended up going back out to get some of that rubbery tape plumbers often use and that sorted it out. I did buy a new rubber seal for between the cistern and seat and replaced that too as apparently they degrade over time.
Touch wood it has been good since, and Greta is obviously impressed with my eco-flushing
Touch wood it has been good since, and Greta is obviously impressed with my eco-flushing