Ok, so that seemed to work. Still not sure why there is no cable to pin 5 in the 12S socket.

Haven't actually taken the caravan for a run yet, but the controller is showing all the right signs.

The word 'catch' in the descriptor for pin 1 - I think this is for an electronic catch for a manual override trailer brake. normally there is a small catch, or physical switch, that stops the brakes from working when you reverse. I have seen some setups where this is an electronic switch that can be set from the driver's seat. So I think I'm safe to use this cable. Must mean that somewhere up from there is already a cable that connects to Pin 1 in 12S, which would save running a cable back in the first place.

Hope this helps someone!