Well, that was easy. Upgraded the firmware using Basemap, put the OSM generic routable map on a 4GB microSD and it just works. OSM looks just as clunky as the Garmin maps on an eTrex Legend HCx, yellow and red and hard to read.
The only minor issue is that autorouting is pretty useless; Swindon isn't even in the towns database and, of course, there are no postcodes (Royal Mail copyright*). Having said that, autorouting was crap even with Garmin CityNavigator maps, with lots of spurious no-through-roads, so I tend to follow a pre-mapped route.
*didn't we once own that? Bastard Tories