UPDATE.
Just returned from a short week in Scotland,
Still on the air springs, but discovered one front spring was fitted upside down, my fault for following the way they were already fitted, anyway corrected that, checked nose weights, altered tyre pressures to land rover recommendations for towing, loaded car and van correctly, and......
Its better than it was, seems to have stopped see sawing,,But it is still horrible to tow with, feels very unstable when I pass slow moving wagons,
The rear of the caravan seems to be sucked into the wagon, meaning correction of steering which then induces a snake, this is at approx 55mph, didnt dare go above, and when cars pass us, it creates the same effects, so not good but better than it was.
Im going to look at the friction pads in the Alko hitch and see if that alters anything, after that if no better the D2 is history, unless anyone has an idea.