If it's from the Quartz Digital range, then it's a very sophisticated, and expensive unit. In simple "tradespeak", lots to go wrong ;o(
If a conventional thermostatic unit, then most likely the cartridge is faulty.
I don't know if these have a cartridge even. I'd be inclined to bet on an electronics fault.
Not something for your average plumber though. I would contact Aqualisa direct. If they're anything like the other top makes, they'll send someone to check it out.
"Stuff" is getting very specialised these days. You need someone with experience in these units.