Update at July 2011
The end cap on the end of the spigot which comes out of the back of the square button snapped off on mine, meaning that the button falls out constantly.
Buttons and Retainers (original part numbers listed are correct) but are no longer available from Triton - discontinued after 5 years.
The parts look like they are available from www.showerdoc.com (0131 538 4343).
Peter, July 2011