Thanks for your question.
If you can establish that you have been an uninterrupted user of the access in question for 20 years then you will have acquired a legal easement (ie. a right of way) over it by prescription which means that the farmer will not be able to fence it off. An easement is a right to one owner to use the land of another.
You can refer him to the Prescription Act 1832 to confirm the requisite length of use.
If he kicks up a fuss and you have in fact used it for 20 years then you will have to see a local solicitor to engage him in correspondence and if he is still not compliant to make an application to the Land Registry to confirm the right of way.
You may consider doing this in any event since the right of way would then be noted against the registered legal title to your property and thus make the property a little bit more marketable or avoid a difficult pre-contract enquiry on the matter in the event you should choose to sell the property in the future.
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