Hi Juergen - I'm unclear as to what could be 'rectified' in the scenario as raised. If a right needs to be granted then it's for the landowner to do that using a Deed of Easement as described. Naturally such a grant needs to be by the actual landowner as you are already aware.
A prescriptive easement, namely a claimed right, can be applied for but the claim needs to cover at least 20 years of use -
see PG 52 for guidance. So if your seller and you have more than 20 years use then it is something to consider if you don;t wish to take the formal deed of easement being offered
Very much something to discuss with your legal rep re options and what happens next and how the claimed against actually granted easement may impact in the future