Hello. I am a solicitor with over 15 years experience. I will try to help you with this.
Has your neighbour reduced the size of your garden by doing this? Dud he do it without talking to you? How long have you lived there? How long has he lived there? How has he justified his view that the wall is in the wrong place? In order to answer your question I need this information. I will be away from this site for some of this afternoon and evening but will look again later today.
He has not rebuilt to wall yet but he intends to place the wall on a different place and will reduce my garden. He is doing the work without talking to me. I lived there for 21 years. He lived there for some 19 years. He claims that the boundary wall was in the wrong place. he has shown no evidence of that. Indeed the OS map show that the boundary wall was in fact in the proper place
The OS map may not be the most reliable indicator of the legal definition of the boundary's location. The wall has been on it's current location for over 21 years I think. Do yo know how long it has stood there? Have you seen office copy entries of the title plan of your property. These can be obtained on line here http://www.landregistry.gov.uk/public/property-ownership and although not definitive may be of assistance.
I am going to be away from this site for the next few hours but will come back later this evening.