Quick report back. The map transfer program took 3 goes before I managed to copy across the whole of the uk to a quest on version 4.0.
It was either because I de-selected bits of germany I wanted across, or I was careful to not muck about with anything while the last transfer was done. When time permits I'll try and break it again and send feedback.
So it's close... Garmin just need to get all the other good parts of mapsource ported now. (The map renders fine in mapinstaller.app, so they probably have most of the really ugly bits done).