Finally, I have a system that works. I've been picking up little nuggets of information from here and there, which have just come together for me.
For anyone else with the same problem, this works for me.
I use oziexplorer to load the PNF's (Personal Navigation Files). This gives me waypoints on a blank map. I then use the ozi route manager to generate a route, using the waypoints in order. I can then export the waypoints to the POI folder on the GPS as a Magellan .upt file and the route to the Routes folder as a Magellan .rte file. This is done with the GPS connected in file transfer mode. It took me a while to realise that the Magellan needs waypoints as well as a route, and this is key to the process.
I have also found that I can plot a route directly onto my GB MemoryMap, by clicking points, then export the waypoints and route as above, using the MemoryMap GPS menu. Soooo much faster than keying marks into the GPS by hand and then using them to form a route!
As you can tell, I am relieved to have cracked this one! My Explorist 210 has a new lease of life and I've saved myself £200 plus as well. Off into the hills this weekend!
Sorry this old hat to most of you, but the Holy Grail to me!