Hi Paul,
My shower had the same problem and eventually the on/off control stopped working too. The problems are linked and it means that there is a little D shaped piece in the controls that has been fitted the wrong way round. If you don't get it corrected then eventually the piece that drives the on/off will split and you will also get the other problem. I can't explain any better as I didn't install or repair it, however, I know this because the Triton engineer has been out and fixed it this morning. I would suggest you get in touch with Triton if it is still under guarantee.
Chris, May 2011