Right gents, a bit more investigation, and I've now got a very, very good idea about where the problem is (I'm reasonably methodical, so I wanted to find out the issue for myself.)
Firstly, this is not a hardware issue. I've started another game as the English, and played multiple turns/battles at one sitting. My method was to save every turn in order that, if the crash happened again, I would be able to pinpoint the exact cause. I was successful, I think.
It's killing the Irish King/destroying the Irish faction.
I played a new game until turn 11 (when I'd reduced them to just the King in terms of royalty) and by this point had a massed army outside the base the IK was in. Save the game. Leave him alone, ended the turn. Happily goes through the opponent's turn and onto turn 12. Reload from the point where I'd ended turn 11, this time killed the King (and thus destroyed the faction), ended the turn, and now it crashes again, exactly at the same point as before, directly after the Baron's Alliance move.
Whether this is enough to point people in the correct direction on the previously posted event log, I don't know, but for now I'll just have to play and tolerate them occupying the southern part of island (not unlike the sentiments expressed by some people in today's political climate, but that's another issue. )
Once again, thanks to the authors: other than this trivial niggle, really enjoying the game.