2 I used https://garmin.bbbike.org/ today to download an OnRoad map image of France, but I don't know how to have my 60CSx run both it and the UK city navigator map, or if it's even possible.[/sub]
The 60CSx will only support one map at once.
The map data file is called gmapsupp.img, in the "garmin" folder that shows when the GPS is put into USB mass storage mode and plugged in to a PC.
To use different maps, you've got to have the wherewithall to replace the gmapsupp.ing by another one containing different mapping data.
So on your PC, you'd have a copy of your existing data called CityNavigatorGB.img, and another file OnRoadFrance.img, take a copy of whichever you wanted to use, rename it to gmapsupp.img, and copy it to the GPS garmin folder, overwriting what was there.
Whether the 60CSx would successfully connect to a phone using a mini to C cable is another matter.
It is possible to have several maps combined into one gmapsupp.img file, but that's not something I ever tried, as my 60CSx got broken before OpenStreetMap was mature enough to use instead of CN (shattered aerial cover, but still works OK). "mkgmap" is the tool used for the merge, as far as I remember.