Thank you Geomen - problem solved.

I followed the on-line instructions for the GMapTool and allocated numbers for FID and PID. This didn't initially resolve the problem until I spotted that the three test maps had all been given the same MapID during conversion to IMGs.

From Post #1 "MapID=<hex or decimal number>", where hex number must be either prefixed with '0x' or '$', or suffixed with 'h' - to manually set MapID, default is a random id.

I thought that the default setting gave a random id for each converted map, but it used the same one for each of the three maps I converted in succession.

Once I changed the MapIDs to unique numbers with the GMApTool all three were visible and selectable in Map Setup. Next time I will manually set the MapID in the command line.