I am sure one of our legal experts, probably Barmaid will be along to guide you. The problem you have is that you need to know accurately where the boundaries are marked on the land registry (or Deeds if it is old property that may not be registered). For modern properties the land registry is usually very clear, but not so on many not so new properties. You can get a copy of land registry documents, but there is a charge (It used to be about £30.00 a couple of years ago)
Quite honestly, if you are going to consider purchasing this property you will have to use a solicitor and this will then incorporate and proper search should bring up queries on things like you mention. If the fence has been up for many years - I think 10 or over - then unfortunately adverse possession of land might come into play.
However, I am no expert.