If you can’t resolve a boundary issue with your neighbour, you can:
Get help from a mediator: you can find one through your local council. Or you can use the RICS Boundary Disputes Mediation Service.
Get help from a solicitor: some disputes will require legal advice to fix. Legal fees can be expensive, so before you go down that route, check whether you have legal cover included in your home insurance. If you do and you have a strong case, you may get your legal fees paid for you.