In order to establish a clear boundary, you should first gather all the necessary information about your land and other pieces of land bordering it.
Go through all the property deeds you were given when the property was acquired. Try and obtain comprehensive and relevant information from the Land Registry, who may have registered older documents although it's likely that you will only receive a vague indication from these sources.
Try to avoid any potential dispute by reaching common ground with your neighbour. At this point, you can enlist the help of a Chartered Surveyor or a solicitor as an extra option.