That the sortie coordinated with a relief force would lead to the besieged losing the fort and fleeing the region comes as a surprise. One might have thought they'd just be forced back in with lower morale. Is it always this way if you lose a battle involving a sortie order?
I suppose that if you have a small garrison, you don't take the risk of a sortie and just let the relief force do its work, and that the alternative for a large army in the fort is to do as you discussed earlier - leave a garrison and then drag the rest out into the region in their own attack, not a sortie coordinated with a relief army?
Not sure if it always is ... as I now know if you just want to fight a besieging force with your garrison simply put the latter into the province outside the fort. In this case, my guess is that my force merged into one (as far as the game engine goes) so when the relief force fell back it took the garrison with it - if so then that is something to watch out for. I think the AGEOD game engine is reasonably logical in its handling of this mechanic its just not intuitive but it seems that having in effect 2 spaces (fort/city etc and countryside) in the same game space forces some degree of difficulty on the game engine.
Now in this case I got precisely the outcome I wanted. After the Savannah disaster I become very unkeen to have a force under siege with no supply wagons, as it is I have the English (oddly as it may appear) exactly where I want them. Of course the normal situation would have been a 2-3 battalion garrison and a decent sized force arriving to relieve them, but I'm still not really getting my garrisons properly calibrated (& its a lot harder in WiA than RoP where garrisons beyond a notional force are pretty useless)