Ralph, while not always correct about the legal rights in a route, this website is not a bad place to start looking for lanes. http://www.trailwise.org.uk/gmaps/gmap.htm

Other options, Ordnance Survey Maps, you are looking for routes like this one http://www.bdcc.co.uk/XMarksTheSpot.htm?g=SD9345583969 this is a BOAT. The problem with OS maps is that they aren't updated regularly enough for you to 100% rely on them, just because a route is shown on an OS map it doesn't mean you can ride on it. It may have been closed as a result of a TRO (Traffic Regulation Order).

The other, and best, option is to join your local TRF group. They will show you loads of legal lanes that you can ride.