As Sudynim has just explained the seller can only sell a piece of land once before it becomes the property of someone else who takes occupation of it.
The Law of Property Act which was passed ion 1925 still applies to all land sales in England and Wales and Section 62 of that act clarifies what is being conveyed unless a particular clause is inserted into the sale deed that excludes this Section of the Act, which doesn't often happen.
Section 62 is very precise about what is being sold.
62 General words implied in conveyances.
(1) A conveyance of land shall be deemed to include and shall by virtue of this Act operate to convey, with the land, all buildings, erections, fixtures, commons, hedges, ditches, fences, ways, waters, water-courses, liberties, privileges, easements, rights, and advantages whatsoever, appertaining or reputed to appertain to the land, or any part thereof, or, at the time of conveyance, demised, occupied, or enjoyed with, or reputed or known as part or parcel of or appurtenant to the land or any part thereof.
(2) A conveyance of land, having houses or other buildings thereon, shall be deemed to include and shall by virtue of this Act operate to convey, with the land, houses, or other buildings, all outhouses, erections, fixtures, cellars, areas, courts, courtyards, cisterns, sewers, gutters, drains, ways, passages, lights, watercourses, liberties, privileges, easements, rights, and advantages whatsoever, appertaining or reputed to appertain to the land, houses, or other buildings conveyed, or any of them, or any part thereof, or, at the time of conveyance, demised, occupied, or enjoyed with, or reputed or known as part or parcel of or appurtenant to, the land, houses, or other buildings conveyed, or any of them, or any part thereof.
On the date that you completed on the purchase, the fence was already erected to define the extent of the land being sold as all that land was "appertaining or reputed to appertain to the land, or any part thereof, or, at the time of conveyance, demised, occupied, or enjoyed with, or reputed or known as part or parcel of or appurtenant to the land or any part thereof"
The fact that the words include "reputed" is sufficient to pass ownership to you.
When the other neighbour bought his land, you already owned property which meant that the developer was unable to sell the area of disputed land for a second time.
You need to inform your neighbour of that fact and explain to him that he needs to look at Section 62 of the LPA 1925 or have it explained to him by his conveyancer.
That is sufficient to establish that you now own all the land