I can explain this process now, but I don't know if it applied as far back as the 1960s.
Adoption of the highway, it's verges and other open spaces left as part of an estate development are covered by Section 38 Agreements. These are drawn up between the developer and the local authority and they explain what standard the developer has to construct the roads, landscaping etc. to before the council will adopt. Adoption does not mean that they take legal ownership of it, merely that they maintain it.
Nowadays, councils insist on financial agreements, often known as section 106 Agreements, whereby the developer hands over a sum of money as a bond, early on in the development, to guard against the developer going bust before the development is complete. This money is then used to complete the roads and landscaping, such that the basic standard is reached and maintenance period (adoption) starts.
It seems very odd to me that the construction standard was reached yet the council say 50 years on that adoption agreement was never reached.
I'd suggest pursuing this with the council by writing to the legal services department at the council and ask to see the s38 agreement for your development. It might start your quest, which I suspect will be a long and frustrating one.
The situation described above is probably not comparable. Jeza probably owns the open land with his/her neighbours and a legal agreement in the land title forces a contribution on each owner to assure maintenance of this land - in this case the council has the contract. That cannot retrospectively be forced onto you.