Question about rebel AI

[QuintusSertorius]

I can't get the game to play. It gets as far as selecting a faction in the campaign, or picking an army, but then when you click start it just freezes,
I'm not sure what I've done wrong

The game never "just freezes". It either crashes or (as in this case) it's doing something. If you break the operation while it's processing, you break the map.

You need to re-patch (or just delete the map.rwm) and start a new campaign, and WAIT. It can take up to 5 minutes the very first time, after which it won't need to do that again.