The best advice that we can give is not to become involved in a row with your neighbour at all. Disputes over land have a habit of becoming very expensive. If you genuinely believe that the fence is not where it should be then ask yourself “Is it that important?” People seem to have a habit of treating their home as their castle and will go to any lengths to show their neighbour where the boundary should be.
Try to keep a dialogue going with the neighbour and try not to make declarations of who is right and who is wrong. If a dialogue is kept open then there is always a chance that the problem can be resolved.
If there is no resolution then speak to a solicitor experienced in dealing with this kind of litigation. Going back to the firm that acted when you bought the house might not be advisable for two reasons. Firstly they might not be equipped to deal with it, and secondly they might have caused it by an error when you bought it!