It's important to note that the title plan map is the legal document that will stand-up in court. The Land Registry calls the ownership boundaries map, the "index map". This is because it's intended to be like an index in a book to get you to the right place, not a precise map with precise boundaries. Over the years the index map has become much more accurate, but it is not a legally binding document in itself.
We'd recommend to buy and review the title plan early on when doing land assessments to avoid potential problems later down the line.