So the cupboard had the carcas and then the outer wood so it was approx 40mm deep... I used a metal knock out box and drilled at an angle into the void behind. This way no cables were on show inside of the cupboard. (Result)
As I was there,there was a handy man doing some work in the garden. He has agreed with the client he will cover the back of the box with some thin white mdf. Same kind of stuff the back off wardrobes are built with. So all in all a very happy customer ??