Land grabbing
Just before anyone gets in to a major legal wrangle SWB my understanding is this. If the land in question (vacant since 1970s??) had been used by someone without secrecy, payment or permission for more than 12 years as if they owned it then legal rights to it are acquired. It MAY be that the new neighbour bought the property from your previous neighbour together with the land in dispute on the basis the previous owners has such an acquired title.
Still agree that the new neigbour sounds a d*ck.