Hmm. I've disabled and reenabled a number of mods without anything interesting happening, but it appears that disabling the one that runs a periodic pcb might have done the trick: it's too early to say for certain because the crash is extremely erratic, but it did seem to run for longer this time and only stopped because I was starting to get motion sickness and quit!
The memory usage had crept up to about 1.5GB since there was no pcb being done, but I have a memory of pcb being problematic on Oblivion too: if you ran it at the wrong time (possibly the same time as it was loading another cell, though my memory is hazy) it would also ctd. Given that FO3 seems to be constantly loading stuff as you wander around, and I've now got even more of it with the BA Wasteland Restoration mod (lots of trees!) I guess the risk of pcb occurring at the same time as something being loaded into memory was quite high.
That's the theory, anyway, but with no real supporting evidence that this has been my problem: now all I need is another unexpected ctd to demolish my theory!