Lowercase2,
I am amazed to find your post. The situation described is exactly what Bovis are currently putting my family through.
Over two years after we completed on the property, and spent money landscaping the garden, building decking in one corner and a shed in the other, they arrived at our door unannounced and told us that they thought the fence was in the wrong place.
(they put it there before we bought the house) and it would have to move so the borders, shed, decking... All scrap.
I retrieved the Title Plan from Land Registry; it matches perfectly the fence line on the ground. The developer says that their Transfer plan is different, and that this is more important.
Totally lost as to what to do.