Network Rail Land Claim

[Suzieq]

My parents purchased this home in 1997. At the bottom of his garden there is a high bank leading down to the railway track. You cant see the track as it is all overgrown. My Father has been there 27 years and has today received a letter from Network Rail stating he has encroached upon Network Rails land and that there are options to resolve this matter, including the potential for renting the land from them. They want documentation if we believe we have a right to the land. The area was also being used by the previous owner. He has also tended and tidied up the land around it, killing weeds and cutting back trees. Can anyone advise us on this matter please?
Thank you

[des8]

Check on Land Registry to see if it is or is not registered Search for property information from HM Land Registry - GOV.UK (www.gov.uk)
and then look at the possibility of making an adverse possession claim

Has he fenced it in?

Have the neighbours treated the ground similarly?