Not to contradict Vanilla, but the situation is in my view potentially more complex. You have a situation here where a builder put up houses, built site boundaries in a certain way, mapped the sites in a different way, and sold them to house buyers. I make the following observation:
A. Look at it from the neighbours perspective; If the builder had incorrectly mapped the sites, so as to register people with less ground than they actually possess, would anybody think it fair of a neighbour to come in taking some of their ground? I think not. It is quite possible that a court would find that the boundaries as built represent the clear intention of the parties, and would order the rectification of the site maps.
B. I would qualify the foregoing remarks somewhat if houses are sold off the plans. In this case, clearly the buyer is only able to go by the map supplied.
One cannot assume that in a conflict between boundary as built and boundary as mapped, the map must always take precedence. It is not a neat, tidy area; A lot may hinge on the individual facts.