In May 2008 I purchased a Satmap Active 10 GPS device. I started with the South Downs Way map card but have had a great many areas added to the card by going to to Satmap’s HQ in Leatherhead.
The Active 10 has been an excellent device and has stood up to the rigours of mountain biking pretty well.
Below are some .gpx files that can be used by most GPS units.
Glastonbury Festival 2009 GPS locations of all the stages and major areas:

an image of the file loaded into Google Earth
The route for the 2009 BHF SDW Randonnée, the locations of the checkpoints and the locations of water taps as .gpx files:
with some tap information dervived from http://www.southdownsdouble.net/I'll add to this section in the future.