Well, good news! I fired up my laptop and plugged the empeg into that, and hey presto, it worked first time! I can only presume that my the RX line on my workstation's serial port has somehow been fried (which as I never use it, I wouldn't have noticed).

The bad news is that my laptop is being replaced (it's a work machine) next month with a new one, which has no serial port. So as of next month I'm unable to connect to my empeg via serial. I know that there are issues with some USB->Serial convertors, but can anyone recommend one that they've had working fully?