Ok, having considered my solution. I have decided to go with a dual flush flapper type of valve, I will fit this today and if it goes wrong I will post here again. No further comments from myself will mean this flapper is good. Picture added of the exact item I'm using, the fluidmaster 507UKK073...
Hi, just to inform everyone on the Wirquin / Jolly Flush type of valves. The problem is that the silicone rubber seal soon fails, we are talking only a few weeks. What happens is the seal perishes very quickly, it starts to bubble up and then the slow leaking begins inside the toilet bowl...