Just found that its not a dual carriageway, just a three lane single carriageway, so the limit is 60mph. (Think my definition of a dual carriageway was a little off).
I think you may have misunderstood the terms, Dave. There cannot be many country lanes with a dividing barrier separating the traffic in each direction.
A dual carriageway (on which the NSL is 70mph) has to have such a barrier. Roads without one, but which accommodate two way traffic not separated by a barrier are single carriageway roads and are subject to a 60mph limit when the NSL applies.
On many such roads (and indeed dual carriageways) lower speed limits often apply.