Gah. After a pile of really easy missions, I stuffed up Chapter 13 (naturally) five times before getting it right. It wasn’t so much that it was difficult, it’s just that I had to get Matthew to talk to Guy (one of the baddies) to “turn” him. This wasn’t as simple as it sounds, as getting them to meet without one of them dying, or one of the other characters (who I hadn’t been paying so much attention to as I was concentrating on Matthew) pegged it unexpectedly from a rogue arrow or something.
So, due to me not wanting to kill off any characters (remember – when they’re dead, they’re dead!), I restarted each time.
Somehow, I’ve now unlocked a side-quest. I wasn’t going to bother with them in order to get through the game a bit more quickly, but I changed my mind last time I had one to do, and so agreed to it anyway. 13X awaits!