I was expecting the 30mm nut to take some shifting, considering the size of it. £6 for a quality 30mm socket, and with a extension bar set to work on it.
It was bl**dy Lose !
Just loosened it slightly and the Oil flooded out.
Hopefully it has just come lose over time. I've tightened it up and it seems to have cured it. Just need a long run, towing the Caravan to check it out !
Fingers crossed.
Jubby