Downloaded CN EU 2015.20 via Garmin Express, which told me I'd have to use Basecamp to transfer the mapping, and even though the Progams and Features says 4.2.5 is still installed, Basecamp fails to start up, just as if I'd installed 4.3.x :(
On the phone to the Garmin EU support guys and a few times of removing the device from Express and reaquiring the Montana, Express decided to show the CN EU 2015.20 update. Which is now downloading :)