The access to your new property does not come directly from a public highway, I assume.
It passes either over someone else's land or (more likely) Common Land, a public bridleway or a (wide) public footpath. In such circumstances it is in contravention of various Road Traffic Acts to take a car up there (if the latter) or the landowner might turn up and decide to deny access (if the former).
Under the Prescription Act of 1830 something, if your seller has been gaining access to his land for over 20 years by going across other's land, a permanent right of access can be established in law and registered at the Land Registry. Your seller has to declare using a Statutory Declaration (a sworn statement) that he has been doing this, and then the rights he has established over 20+ years can be transferred to you as the buyer.
If he didn't do this, you might well find your right of access suddenly disappearing.