We seem to get information from you in tantalisingly small, bite-sized chunks on this case but without getting the bigger picture.
Putting the bits together, the gist seems to be that you are using a legal process to determine a physical boundary between you and a neighbour. You have gone down a route of trying to use OS maps dating to 1925 to show where the original boundary was. Since the matter escalated between you and the neighbour (involving solicitors), the neighbour has erected a new fence, cutting a parcel of land about 2m by 20m from what you regard as 'your' land. The neighbour maintains that this fence is erected exactly on the line of a historic old fence that he found when he went to erect the new one. You are hoping to use adverse possession to determine that the physical boundary aligns to where you have most recently believed the boundary lies.
If you are going down the adverse possession route, one of the things you have to demonstrate 'factual possession'. There was a landmark case in 1979 (Pye v. Graham) where the judge provided a statement as to what constituted factual possession, which was later (in 2002) approved by the House of Lords.
One of the best pieces of evidence is that a fence existed between the two.
You've never told us what the state of the original boundary was (that you believe demarcates the two parcels of land) - whether it was fenced, hedged, ditched, or what. Also what cultivation or other management you have done on the land now separated on the other side of this new fence. But it seems to me that that will be a factor in determining the extent to which you can show 'factual possession'.
These cases can rack-up big legal costs - far more than the worth of the land under dispute. I do hope you've considered that. As always, your legal people have access to all the evidence whilst we only know what you have told us.