Dear Steve - it reads as if you need legal advice here but from a registration perspective then our PGs 52 and 62 deal with easements and how you might apply to register same
Land registration: Practice guides - GOV.UK
Land ownership you can check online or by post if it is registered. If it's not then again legal advice is needed to consider what evidence you/the neighbour have re ownership
Search for land and property information - GOV.UK and then our blog if unregistered
Searching for the owner of unregistered land – HM Land Registry