Checkout this link:
http://www.showerdoc.co.uk/shower-spares/triton/Opal%209-5kw.html
Sounds like a faulty thermal cut-out (No 6 on the diagram) £19.33 inc tax
This company is great, and their parts are very competitively priced. Usually delivered next day. If you are confident you could replace this device yourself.
John
November 2008