Thanks for all the replies, most useful before I start this exciting wiring mission !
For reference, I've located two pre-installed unused plugs from the wiring loom, without removing any bodywork etc.
The first is above the RHS radiator and looks to be power ( believed to be the feed for a second Aux power socket) and the second is tucked into the RHS metal framework for the front end. It has a spare tail emerging from a one into two join plug, the other side of which goes to the Sat NAV mount. Not sure what this is, possibly not power ( a blue and a white wire).
Think Ill be using the Autocom relay from the battery, with a trigger wire for my comms system power...