Just in case anyone else is trying to find a way to get Ordnance Survey maps or similar onto the Nav V, this is sadly the official reply from Garmin:
"Thank you for contacting Garmin Europe. Unfortunately the GB Discoverer maps are only compatible with Outdoor devices so you won't be able to use it with the BMW Navigator V."
If anyone knows any other way to see OS maps on the Nav V I'd love to know.
Max