I tried a few games with the big earth map but it eventually got a bit too familiar. I play the Capital kill variant on big maps and it was too easy to guess where the opposition(s) were going to be.

Anyway, on changing back to a random world builder, the maps seem 'different' to what they had been. The terrain seems different and I frequently start out in the only city on a small island. How can can I reset back to an out of the box setting without losing my progress to date?

You can manage the setup configs, there is a last config and you can delete that. The name should be "lastConfig".

Do this with the "Load Config" menu - there is an option to remove. You may have to go back to the main menu and reload.