It took a while but my dilemma has now been resolved.
It appears, from my experience, that there is something specific within the Garmin update process for NT mapping that prevents you from overwriting the primary mapping with an update from another region.
Up until last night I was stuck with outdated complete mapping of the US and only partial coverage of Europe on my 2720. However, thanks to two kind souls (both of whom signed up to post an answer) I can report that it is now possible to perform an update of the primary mapping and effectively convert a US import 2720 to a full European version (well, as good as anyway).
See the thread on SmellyBikers forum here
Please note that I have done this with mapping that I have paid for an unlock code for. Garmin, no doubt, knew that this conversion technique exists yet refused to help me saying it was impossible to convert the primary mapping from US to European. Smacks of protectionism IMHO.
Please note that this method is only really useful for Garmin NT units that lack a mass storage mode, namely; Quest II, StreetPilot 2720 and BMW Nav III. For the likes of the 2820, Zumos and Nuvis, these have a mass storage mode that permits access to the files on the unit via USB making the process much easier.