How can I fix my Triton Opal electric shower?

After showering, I press the 'off button' on the shower. The water does not turn off at all but runs cold. I can only switch it off by the cord. I telephoned Triton service desk who said I needed a new switch which was delivered a few days later. When a friend recently offered to put the new switch in for me he said it could not have been the switch but something wrong on the cicuit board. Has anybody had this problem? I can't give the model number as the number and kilo size is now rubbed out!