Just as an update, after much faffing around I got it to work.

I dug into the PC side of things a bit more and worked out I had the 32bit version installed, which while it probably wasn't the problem likely didn't help. I downloaded synergy-v1.8.8-stable-Windows-x64.msi from Sourceforge, re-installed and got it to work.

I think the problem was on the PC side - Synergy was perpetually in "Starting..." mode (and it still says that) while giving out an error. Whatever I did, I'm not changing it!

Thanks all.
Sam / Bifford