OK, so now it's all gone horribly wrong.
I couldn't see any option to disable a network port. I downloaded the latest BIOS from the manufacturer, although I'm sure I was already on that version (it was almost 5 years old, the machine is about 7).
I followed the instructions very carefully to install this version, and that went through without any errors or problems. And that was the last time I saw or heard anything from my device
So I've tried everything I know to get it going again, but there's nothing happening. No beeps, no display, nothing. I've disconnected all the disks, taken the CMOS battery out, tried it with no monitor connected, Nope!
So now I want to get my data off the disks. I was using Greyhole. I have a USB to ATA/ATAPI device. I've plugged that into a Windows PC, but nothing shows up in File Explorer. I'll have a look in the Wiki and this forum, and if I can't get it working, I'll start a new thread.
Thanks for your help with this job! Maybe I'll try again with another machine, I'll make sure that it only has one network card/only one enabled!
Steve