There was a Boat sign erected so I assumed it had been legally changed, however it still appears on the List of Streets as an "Unclassified highway" and can't find any record of the change in the Derbyshire CC records.  
OK in this instance mapping ground truth was probably wrong without further verification, so I'll change it to unclassifed_highway, and speak with my TRF friends up there to shed light on the sign, and confirm if still there.
Cheers