You can't transfer these maps between Garmin units like that. OS maps purchased from Garmin are locked to the SD card they were supplied on or if downloaded the first device they were used on. The only way to do it is to purchase them again for the new unit.
I would investigate using OSM based maps that have the features you want in the OS maps as they can be transferred and updated. You can even make them yourself so you have full control over customising the look and what features are in them.
Thanks @wongataa Can I do a decent off road map using ODM though? With the off road tracks? What I could do is use the standard Garmin map with contours and use the (pretty decent) OS Maps app on my mobile as a backup.
The Garmin cycle maps are also based on OSM data. Go to the OpenStreetMap website and see what tracks are in the areas you are interested in. There is no reason why an OSM based map cannot be decent. If there are missing features you can always survey them yourself and add them so when you next update the map on your Garmin they appear.
