Got this sorted, so just for the benefit of anybody else in the same situation...
First point is that Garmin Express is a PITA. Tries to do everything automatically and if it doesn't just doesn't bother telling you. Hopefully it gets better because it is the only tool they will be developing.
On PC
Task 1: Get the maps onto the PC so they can be read in basecamp without. the Zumo connected
Solution: Use Map Updater in preference to Garmin Express, but hopefully in time Express will do the job as well.
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http://download.garmin.com/lmu/GarminMapUpdater.exe
CLOSE Basecamp. Not sure this is necessary, but it was necessary to do a restart later on.
Run MapUpdater (runs from the downloaded exe file)
In my case when I try to download it says insufficient space on device (it can't see the 8GB SD card in the Zumo 550 - apparently it can in some other devices). Then you get the option to install to Computer, Computer and Device, or Device.
Select Computer as the target.
Tell it where you want it to save the files.
Come back a long time later.
If all went well open basecamp and the maps will be there. End of story.
In my case they were not. It turns out that both MapUpdater (default) and Garmin Express try to save the files to c:/ProgramData/Garmin/maps (you must have selected 'show hidden files' in folders to see it). I selected aother folder in Dropbox. This shouldn't be a problem. The install should put a link in c:/ProgramData/Garmin/maps to the alternate location. For some reason it didn't in my case (it may have been because Basecamp was open or for some other reason the folder couldn't be written to). The solution is to plce the link manually.
It looks something like this
C:\ProgramData\Garmin\Maps\City Navigator Europe NT 2013.41.gmap which is pointing to the folder where the maps are stored.
Did that, restarted basecamp. Maps appear!
Task 2: Put full maps on my new SD card.
If you try to do this with Garmin Express or MapUpdater you will be told there is insufficient diskspace. Neither can see the SD card in a Zumo 550. Apparently it can in some other devices.
Solution download and install MapInstaller
http://www8.garmin.com/software/MapInstall_403.exe
Once downloaded run and follow the prompts. It will only see 4GB on the SD card (I presume it is a windows FAT issue). There are work arounds, but the full Europen maps are only 2.8GB anyway.
Come back a long time later...
Finally I have maps on a Garmin and on a PC. What a load of cnuts the Garmin desktop app developer are. I'm bloody exhausted.
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