So i've compromised my cold-storage wallet?
I'm probably right then in thinking the only thing to do now is create a new cold-storage wallet on my offline computer and use it to generate a new watching wallet on the online computer?
Damn, what a pain.
Thanks a million for your help though!
Yes that's right. There's a guide here if you are interested:
http://docs.electrum.org/en/latest/coldstorage.html