Try fitting a new flush valve.
I, (with no plumbing experience and guided only by youtube) replaced my flush valve and fill valve when i couldnt find a replacement flush button to fit my existing valve.
Get the kit from Screwfix, its a fifteen minute job. Good advice, that I didn't have, is always replace your washers as they will never seal properly again. So arm yourself with some new fibre washers and get a new doughnut as I had to take it all apart again and change mine when I realised that the old one was leaking after I put it back together again (mine is a close-coupled jobby).
Oh, and if you're using those drop-in-the-cistern blocks, stop using them.