I've done a bit of reading but just want some input on this.
If a Nuvi 2xxx already has a map in it and I want to add a second map (so I can choose depending on which country I am in at the time)
is it sufficient to add \Map\[name].img to the /.system folder and put the new map and associated files in there?

Yes! in the root in a directory \map you put it with any name ! Just has to be unlocked and your firmware to support unlocked maps

Thanks Kolo.
How would I tell if the device is unlocked?

You will discover if your device cannot recognize an unlocked map or have a message like Cannot unlock map

Thank you again Kolo.
I managed to get it sorted - now have the new and the original maps working.