Hi Sadie,
Bellway have made a mistake - they either misinformed you that the land was yours when it wasn't or submitted the wrong info to the land registry. Either way you have been mis-sold and it would be reasonable to expect some kind of compensation. You need to work out the value of the 15 foot - if this is less than £5000 you can go down the small claims route.
Which offer a service that was recommended by fellow member MikeMurk.
I hope this helps
Tony