Galaxy Aqua 1000 has begun cutting out under pressure?

I can turn the unit without problem, and I can turn the little heat setting knob onto each setting with no problem. However, as soon as I turn the big knob in an effort to get water flow from the shower head the unit cuts out completely. Water will still flow from the shower head but it will not be heated, nor will the heating element be working, nor will there even be power being fed to the unit as that cuts out too. I've never had this happen to me before so I wouldn't even know where to start looking, especially as I'm not particularly practical and had the misfortune of suffering an almighty shock some years ago which rendered me absolutely petrified of electricity and I wouldn't attempt any remedial work unless someone could point out exactly what the problem is and how can I can correct it, or even circumvent it.

Any help would be hugely appreciated guys.

Thanks in advance.

A smelly man in need.