Thank you for your coments, i am trying to gather as much information as possible to try and rule out wasting money, but all comments are welcome to try and put the bigger picture in place. The pathy runs fine with van up to 56-60mph its then that it goes pear shaped. Thanks and keep them coming.
These type of problems are always difficult to solve.
A few years back I owned an Abbey Oxford which I towed with a Vauxhall Carlton, and was very pleased. When my company car was changed to a Granada I had a similar issue to the OP. I tried everything, eventulally I tried towing someone else's van, and there were no issues!!!
So I changed the Abbey for a Eccles, same layout and weight and had no issues.
The only conclusion I came to was the wind flow over the car upset the Abbey but not the Eccles for some reason. The Carlton did not upset the Abbey.
Thank you for your comments. I have found out a few things since asking for advise. The pathfinders suffer from mis wheel alignment problems which will normally only be found when towing. On the advise of a car repairer and a tyre garage and practical caravan magazine, i found a garage which does electronic wheel alignment. This process looks for heal and toe of each wheel when the vehicle is at stand still and by rolling it backwards and forward. A computer print out then shows you the percentage of adjustment required to bring all wheels back to the manufactured positions. The garage adjusted all the wheels up and the difference in towing was instant. Don't be mistaken this is not tracking or wheel balancing but wheel alignment. The garage told me that the audi Q7 is the worst for this problem followed by the land rover defender, but any vehicle can have this problem simply by hitting a speed bump or hitting a kerb to hard or quick.
I have another caravan trip planned in may so i can monitor the progress once again and report back.
Regards Phil