Thanks everyone... but I officially give up (well for tonight anyway)

I moved all the existing files, as mentioned above, into a backup folder.
I renamed the TESTED AND WORKING img file to gmapprom.img

Powered up the GPS and.... nothing. It sees no map at all.
All I see on the screen is a small c copyright symbol. No map listing at all.
When I actually view the map it's just the basemap.

So it's not the img file, it's not the SD card... there's something evil about my GPS!!!

I gotta run but if anyone has any ideas please post them here and I'll maybe give it one more shot tomorrow before conceding defeat.