Hi Mellis, thanks for your reply.
You're right about the communication - doing the loop-back test gives intermittent results, giving back some characters typed in, but by no means all. Certainly no visible damage, and I'm not aware of having done anything which might have zapped the wrong pin (though I guess anything's possible!)
Do you have any other suggestions for things I can try, or is it time to buy another Mega? Seems a shame given that the rest of the board appears to be functioning okay.
Cheers,
Jo
(Another possible symptom: with just the board plugged in on its own, the TX light is now staying lit solidly except for when trying to load something, which is a bit weird. In normal operation of my setup I expect TX to flicker in response to inputs on A0-A7, and otherwise to remain off. Grounding the analog inputs makes TX go off. So there's some kind of unwanted interaction happening in there somewhere. *Sigh*)