Having failed to find the option in the GIMP to create colour mapped PNG files, I have just had to discover a couple of handy commands...

• pngnq - tool for optimizing PNG (Portable Network Graphics) images
• pngcrush - optimizes PNG (Portable Network Graphics) files

The first of these takes a RGB PNG and gives back a colour mapped (by default, 256 colours) image.

The second takes those images (or RGB format ones too) and crushes them by about 20% (on the images I have been using today).

So the 22k file that GIMP created is now about 9k!