We have three toilets and over the years, I have replaced all three dual flush units many times. Water wastage has been the problem as they are plastics construction and not built to last. However, reading that nationally, these dual flush units waste more than they save, recommended a single flush.
Managed to buy an Opella single fflush unit and as I am about to take the cistern off the wall will also change the fill valve and doughnut. I usually coat the adjoining parts with washing up liquid and any threads with plumbers' grease (Silicone) so they fit well and are easier to remove.