Disco 4 (LR4) Towing Electrics (12s) Problem

We took the caravan away for Christmas & on arrival noticed "scorching" on the 12s car socket & the plug from the caravan. Looking at diagrams I believe pin No. 6 is the supply to the caravan fridge via a relay (read somewhere there might be 2 relays 1 in the car & 1 in the caravan) and pin No. 3 is the earth return to the car battery or metal.
The scorching is on pin No. 3, the towing electrics were installed a couple of years ago by a local auto electrican's along with a bolted on towbar, the only problem experienced previously was after replacing an older plug on the caravan lead with a new one. I noticed a small scorching and on closer inspection put it down to a loose connection and cleaned the socket & pin then "nipped" the socket to tighten the grip on the pin all been ok since then. My worry is could there be an earth problem or just a bad plug, I'm not sure if the car socket can be changed if it is part of the harness, and could still be under warranty although there would have to be a harness fault diagnosed to have it swapped out.
Can't find any information in the handbook ref alternator output, incase there could be a spike.
Traveled home with the 12s disconnected just in case.
Any info would be much appreciated,

I think all the fuses for trailer sockets are behind left hand rear boot cover.
Suzies and sockets are a royal pita on trucks and just do not last nowadays and I would guess cars are the same.

I would change the sockets and suzie to be on the safe side.