The present owners are adamant that there is no right of way.
It may be sensible to copy the conveyance dated 1972 and write to the owner of the servient land enclosing a copy of the 1972 deed.
You can point out that the granted right of way is a Legal Interest over his land that you own, which was an over-riding legal interest when he acquired ownership of the land.
When you bought your property the law considered that you bought the Legal Estate of the physical property, which included the land and all building erected on that land, and you bought a Legal Interest over the retained land owned by the seller in 1972. The right of way was the easement that the law recognised as a Legal Interest over land which was conveyed to you along with the Legal Estate.
The Law of Property Act 1925 is still the relevant statute in 2017, so you need to make the new owner of the servient land aware that he has no right to stop your access over the route that ought to have been identified on the 1972 conveyance plan.
This is what section 1 of the LPA 1925 stated, with some explanatory words I have added in bold:
1 Legal estates and equitable interests.
(1)The only estates in land which are capable of subsisting or of being conveyed or created at law are—
(a)An estate in fee simple absolute in possession; "normally referred to as the Freehold"
(b)A term of years absolute. "normally referred to as the Leasehold"
(2)The only interests or charges in or over land which are capable of subsisting or of being conveyed or created at law are—
(a)An easement, right, or privilege in or over land for an interest equivalent to an estate in fee simple absolute in possession or a term of years absolute; "The Legal Interest that was conveyed with either a Freehold or Leasehold title"
Despite the fact that Land Registry had failed to record on the neighbour's register title that the land was burdened by a right of way, that was the over-riding legal interest that had existed since 1972 and cannot be removed or obstructed without a legal deed of extinguishment being executed by each party.
You need to make the neighbour aware of those facts, as he is in the wrong and you are in the right to demand free access over a legally granted right of way.