Sussamb - how's this for a hypothesis?
V1 and V2 maps are at different "resolutions" - by this I mean that as you increase magnification in Basecamp they switch from 250000 to 25000 format at different zoom levels. I have noticed that if the map display on the Etrex contains V1 and V2 mapping, it starts by showing only the V1 tile but as the zoom increases it suddenly switches to showing only the V2 tile. My theory is that the Etrex software cannot cope with showing different resolution images on the same screen.
Today's walk also involved a V1 and V2 tile. I cured the problem by right clicking on the V1 tile file in the overlays window in Basecamp and selecting
Create Birdseye Imagery from Selection. This highlighted the map tile in the map display in orange and opened the normal
Download Birdseye Imagery window with the title of the map tile already filled in but the
Next button was greyed out. I added the suffix V2 to the filename which ungreyed the
Next button so I could complete the download. I completed by deleting the old tile file from Basecamp and replacing it with the V2 version. I then transferred the map files, tracks and routes to the Etrex and it worked just fine.
Trouble is that this means everytime I want to do a walk that involves V1 and V2 mapping I will have to rebuy mapping I that I have already paid for which seems a bit of a swizz.