See the OP .... The 6mm² T+E currently goes from CU to a switch-fuse and, as you say, would be inadequate for the new loads. However, the feed from that switch-fuse to the outhouse is 16mm² SWA, and probably just about adequate for the proposed loads.I thought the existing was 6 mm and in-adequate for the loads being added to the building's electrics.
Kind Regards, John