If I remove the cover from the shower & fit the knob direct all works perfect , the shower is fitted to the wall& is level the cover is fitted properly and I can see no damage. Just bought the house so don'tknow the history of the unit although it looks quite new, any ideas?

Seeing how it is that we don't usually work on electric showers here in the USA... YET, I'd have to advise you to contact triton directly. Contact technical support at this link:

Triton Showers - Electric Showers, Mixer Showers, Power Showers, Digital Showers, Bathroom Accessories (http://www.tritonshowers.co.uk/)

Good luck!

Mark