Well, the news is that the old hard drive has given up the ghost. However (and I don't even remember doing this!), I have another hard drive which happily connected to the PC, and I found I had Amos Pro and the CMED source code on there. It booted up fine in WinUAE.
But... (and it's a big but) ... when I loaded the CMEd source code, I got an 'out of memory' error. I tried lots of different configurations but none of them worked. So now I've joined an Amos forum site so we'll see what happens.
Quite frustrating, but hey, at least I have the source now.