When the loo in my wife`s bathroom stopped flushing I found the plastic diaphragm had perished but instead of replacing the "twist handle / syphon" system with the same model I bought the "drop valve" type via Screwfix. It fits the same hole in bottom of cistern and the beauty is any future replacement will just require twisting the internal tube (like a bayonet fitting lightbulb) and fitting the new bit. No need to disconnect the cistern from the pan. It also acts as the overflow!
Fixing the push button part was a cinch. I removed the original handle - leaving slot in end of cistern - rubbed the glazed opening with sandpaper to clean and key the glazing then used a dowel of similar size to the push-button fitting but with some kitchen foil wrapped around it. Mixed some of that jam roll-type Plumbers Mait putty (Screwfix or B&Q) and put some in bottom of the u-slot then positioned the wrapped dowel and more PM moulded into the slot and around the dowel. Some smoothing blended it to the actual cistern for a neat finish. A couple of hours later I pushed the dowel out then peeled the foil out to leave a solid and perfect fit for the button assembly.
Works great - AND I think the drop valve gives a quicker and more powerful flush of water than that via a syphon. I`m going for the same system in my own bathroom - albeit as original loo and fittings. EyeTry