I use Oruxmaps on an Android phone to display Ordnance Survey maps, Open Street Map (Open Cycle Map) mapping, or Google maps. This application can work with online Google maps (3G connection required) or (much more usefully) with offline maps created in advance with Trekbuddy Mobile Map Creator. It might look complicated but the process of creating a map and exporting to your phone is very simple.
Here’s a screenshot. Andy, could you use the Garmin format (KML) shown here to export to your unit?