Thanks SO much for the suggestions, everyone.
Bathsheba, I haven't tried anything yet - I decided to wait til Saturday so I won't have to go to work with a sore finger.
Lullabyblue, As unsentimental as it sounds, I'm not too concerned about the ring itself (it's plain gold and shouldn't be difficult to fix), it's more the fear of having something sawn off my finger! Anyway I'm relieved to hear that you've had it done and survived!

I'll try out some of the ideas this weekend and let you know how it goes.