Why do the diaphragms in my Vokera XL80sp keep perishing?

Ive had the boiler for about ten years now and its been reliable apart from the main (large) diaphragm in the two way valve, this is in the service kit and I have to replace it about three times a year because it simply rots and splits causing the central heating and the hot water systems to flow into each other. The system is filled with Tower XXX inhibitor and was mixed to the correct ratio. The entire system was flushed and cleaned about four years ago. The boiler lives in a cupboard in an upstairs bedroom and is protected from damp and cold etc. The central heating and the hot water are never run at maximum heat, I keep them at about 75% (an old habit from a past occupation) in the winter. The Central heating being isolated in summer. There is no sign of debris in the water when I drain it and the gauze filters are also free of debris. Although we live in a hard water area, there are no signs of this inside the boiler. I only ever fit geniune Vokera spares. The gas side of the boiler is serviced by a registered gas engineer and this is up to date.