First Steps.......There are a number of sources of information, and often a Solicitor is the first call. Unfortunately this is when the money really starts to disappear, read up on the subject first and consult a wealth of material that is out there:-
www.rics.org this will direct you to the consumer guide for boundary disputes
www.landregistry.gov.uk - this is the primary source for finding your title documents, and the best item to read is Practice Guide 40
Important Points of Consideration
Before you go and put you foot in the neighbours' door claiming adverse possession or boundary encroachment based on a glance over your title documents, and your knowledge of the boundaries, there are certain points to consider:-
1) The red line on your filed title plan is only an general indication of where your boundary should fall. It is a 'General Boundary' and there is no way to quantify the degree of variation between this red line and the fence running through your back garden. It is best considered as a starting point for locating your boundary. As a rule if thumb if there is a boundary feature in the rough area of the red line on your plan it probably is the boundary.
2) The boundary on your field title plan is know as your legal boundary, however the boundary running through your garden is the physical boundary. In an ideal world these would be as one, but in reality this is almost never the case.
3) When considering the position of a boundary even the worlds best Chartered Surveyors (dare I say us?) are reliant on preexisting information to base their findings. This is a very grey area as many title documents were prepared on older Ordnance Survey digital mapping which is constantly being updated, therefore your Surveyor will have to bring the existing title documents onto the latest mapping, this process will inherently introduce a degree of error which cannot be avoided. As a general rule of thumb Ordnance Survey mapping is accurate to around 0.5m (but it is getting better) so you cannot simple assume this to be correct and ask it to be set out then act surprised when it doesn't agree with the true position on the ground.
4) How important is that final 10mm?? Most boundary disputes are driven by psychological factors such as greed and principle. Often people try to claim land which at the point of purchase they may not even known they legally owned, but on closer inspection of their filed tiled plan they discover that they may be entitled to a little bit more instead of accepting the 'sold as scene' philosophy!
5) It is really worth a dispute with your neighbours over what is really a trivial issue in the big scheme of things.
6) As a Chartered Surveyors the position of your boundary will not cause us to lose sleep, whilst we may be comissioned by one party in reality our only concern is getting the boundary right even if that is in disagreement with our client. It may seem unprofessional but we really don't care where the boundary falls - it is just a line to us.
7) The initial transfer document in the creation on of a title will in general be the definitive document of comparison even if it is disagrees with the final field plan. Plans do get drawn wrongly as a result of operator error, changes to the base mapping, even the quality of the original document.
8) Dimensions are the Chartered Surveyors curse. A dimension is all well and good, but it is only as good as the mapping it is based upon, and knowledge of the point it was measured from. Times change, features change, but the dimension is still based on a long distant concrete post.
Life is short, and whilst we are ever thankful for the work boundaries generates, do you really want to waste time and effort over a small amount of land?