Thanks for the fast reply.
In the meantime, I added GMAPSUP3 to the SD card and that too worked perfectly! It appears I can add as many maps I like as long as I have space.
I also tried to add one of those maps to the internal memory, called it GMAPSUPP and that too works fine.
So in summary it looks like all the issues I read about names and formats do not apply to this model as (like you say) just adding .img files either on the internal memory or SD card is all you need to do.

I had seen that post and the GMAPTOOL - looked a bit complicated though I had a quick go at downloading it yesterday and didn't work - will try again now.
DO you have a link for downloading GMAPTOOL?
Also, in that post you refer to patching BASECAMP - is that required too? Thanks again...