but also have gmapdem.img on the SD card a 317.8 MB file dated 10.01.2012. Should that be there? is it a worldwide basemap if so what is gmapbmap which I see is also referred to as a DEM?

Thanks.
gmapbmap.img is a basemap containing DEM data showing in the basemap only.
Gmapdem.img is DEM data map for your CN detailed map