When was the last time the map was updated in BaseCamp. As Baldock in Hertfordshire has had a bypass for some years. I have just purchased the Garmin Nuvi 2798 and downloaded the software and still finding my way round the unit and program.
BaseCamp is designed to work with many different maps. The Global Map that BaseCamp comes with by default is maybe a little older but is just a basic map to get a user started. It is recommended that you update your device to the latest map, plug in the device, and use the map that has been installed on that device.
When you update your nuvi's map there is also an option to download to device and PC, doing that will allow you to use the detailed map without the nuvi being connected.