I done bought a gps navigator type thing, to geocache on my commute and when I’m sent to the sticks, and more importantly to not get lost on long bike rides. In particular, long bike rides at night with very few other people. The prospect of getting lost at 3am in Suffolk and needing to use a map is not entirely appealing, mainly the 3am part. So I treated myself on recommendation to a Garmin eTrex Vista HCX, and then wanted to put open cycle maps on it, for cheapness and convenience.
Problems on the way: the UK based site seems to want to sell downloads of the map. Its freely available as an img file and for adding to garmin mapsource (weirdly, being open source) from the here.
I then foolishly wrote it to an SD card instead of copying it across, mainly because my only experience with img files is putting operating systems onto sd cards for a raspberry pi. But you can just copy it across. If you do, put it in a folder “Garmin” then it’ll be read, and this bits important apparently. A lot of forum posts say to keep the title as it is, but this led to the vista recognising there being map files on the sd card but not allowing them to be selected. Change the name form “gmapsupp” to anything else, and hey presto working. Oh, and should you be using something without a cd rom drive, download basecamp from garmin, then update it with mapsource and bingo.