The property is currently being sold as Leasehold. Rightmove lists a freehold sale in 1998, so it is possible that it was simply the sale of the land, not the property and that the owners have indeed lived there since the 60s.
Of course, would make sense. Obviously. Rightmove info is a very blunt instrument.
I remember when I was first buying and had no idea at all about leasehold/freehold. I got enormously excited to find a flat within my (tiny) price range in Knightsbridge before a wiser person than me noticed that it had three years left on the lease and the renewal cost was estimated as £1.5m.