> It may not be the answer you wanted, but it sounds like an answer.

Yup - I can confirm that that was indeed the problem. Luckily, almost
everything else on board is 4.5V tolerant (just about... I might be
stretching a few bits and pieces, but I didn't let the smoke out of anything
when I tried it after checking datasheets).

Unfortunately, I'm using 3.3V SmartMedia, which I doubt will want to talk to
my PIC at 4.5V.

Doh! :(

Thanks for your help.

Kieran