I’ve changed the attributes of the access road from the A3400 to Edstone Hall to allow through traffic up to that building only. This will make traffic following your routing from Langley Road to go around to this entrance instead (if they wasn’t already). An unconventional fix but it should work. Further the postcodes in this area have been checked and are all correctly located.
You could try contacting other map data providers such as Waze, Tomtom/Teleatlas, Google, Apple, Ordnance Survey.