After much research on GPS units, my initial conclusion was you can either pay little for map-less low end units, or ten times more for one with decent maps. However.
You can buy a Garmin Dakota 10 for less than £100 quid which comes with very basic mapping, but there are websites where you can download open source mapping for free, and I mean maps with roads, trees, mountains, contours etc. Because the Dakota 10 has some memory you basically install the maps and you're good to go, for cheap.
The maps are so accurate the unit will tell you what side of the pavement you're standing. So not too shabby.