just for clarification, the picture I posted was just to show how much space I have between the plaster and the brick wall I prefer not to chase into. That double metal box is already now occupied by a double socket. I plan to put a separate 1 gang box left or above.
Agreed on careful handling of CAT6 cable, they don't bend easily and the outer sheath quickly get kinks. Thanks for all the responses so far, really good input. I will order all the parts now and then hopefully next weekend get on with the job.