Water flow on Triton Opal (1) goes cold but does not stop when turned off?

I have a Triton Opal (mk1) shower. When the (on)/off button is pressed to stop the shower, the water goes cold but keeps running, turning the power isolation switch off and back on stops the water. Any ideas? If not, anyone know what other unit is available that has the same backplate layout as this shower unit. Thanks, Rob