Modifying the default Garmin basemap
Several of you asked about using open Street maps (OSM) on your Garmin, to replace the frankly rubbish Base map that most models come with. If you Google it you'll find more detailed instructions on how to create your own custom maps, or if you have space on the device i.e. it has an SD card, copy the entire OSM GB map (about 600MB). Detailed instructions here.
To use it you need to replace the existing default 'gmapbmap.img' file in the 'Garmin' directory on your Garmin device, which you can do when it's connected via USB to pc or Mac, with the new map file: East_Anglia_gmapbmap.img
It's best to copy the original to a different directory on your pc to keep as a backup, then copy over the new file onto the Garmin and then rename it 'gmapbmap.img' so that the device will load it on startup.
You'll know if it's worked when you restart the Garmin since it will come up with the OSM info when it loads the map.