The earth cable would just be for the shower. The earth on the 10mm T&E cable is not the same size as the live & neutral cores. The rest of the bathroom is already bonded.
I am going to fit a new electric shower, 8.5 or 9kw, not bought it yet. I am going to use 10mm T&E cable. Do I have to use 10mm earth cable as well or would 6mm earth cable be ok.