Problems with my Power Shower

Charlie007_2

Hello,
In desperate need of some help, not sure if its electrical fault or plumbing issue.
Bascially problem arose on Saturday night, turned on shower okay but after I had showered I tried to turn it off but the water kept on coming through. Eventually had to turn off the entire water supply to the house. The following day I managed to get some friends advise that said my system was fairly common in new-house's with ensuites, i have an unvented system or something. Anyway I have managed to isolate the water supply to the shower, but how do I go about fixing my shower problem.
Its a Hydramax built in Shower system.

Please any suggestions as to what the cause maybe ??:(
regards