any update on this?
I was looking at the sale details for the plot - and a big thing stood out, its sold in 2 lots and the lot affecting your land has no shooting rights with it, the other one does.
It states they were reserved to a previous owner - now that would scream to me, the access was given for shooting - IE the land was all in one ownership in the past, and the shooting rights were retained on paper - and the owner of said rights, put in deeds of access to get to thier shooting.
As you say your previous owner lost their deeds, its quite possible you dont own the shooting rights to your land - a convinient loss for the previous owner - and someone somewhere is sat with paper proving their rights.
Now this is the bit that could help you - Sight of this access may show its for shooting, or other specific purposes, so they may not be able to put a track in.
HOWever if the lot is sold seperate from the other lot, they will have ONLY the access across your land - and in that case, it is most likely the access was granted for general access - - Your best bet is to try and get them to buy a strip off you down the side of the field for easy straight access - and dont charge lots - be far and charge the value of the land in the area, and you should get a peaceful settlement.