Start by reading some general information, from the Land Registry, about boundary disputes:
http://www1.landregistry.gov.uk/assets/library /documents/public_guide_019.pdf
Having read that, consider whether you think it worthwhile to take a look at your neighbour's title plan. It's available online for �3. (It might also be worth taking a look at the title register. It will cost another �3):
http://www.landregisteronline.gov.uk/
If the dispute can't be solved without formal adjudication, you'll require the services of the Adjudicator to HM Land Registry. You'll almost certainly need a solicitor for this but it would be best to know as much in advance as possible, so follow the relevant links here:
http://www.ahmlr.gov.uk/
Chris