Shower wouldn't switch off?

I have an Aqulisa digital shower, after showering the other day I switched the shower off and the water didn't stop, I had to turn the electric mains for it to stop. Last night I was woken by the shower water was coming out and again the only way to stop it was to switch off at the mains. The shower doesn't switch on now either. What could be wrong and is this a job for a plumber or electrician?

Steve
December 2007