Traditionally it is seen as part of a 'contract to marry' and should be returned. However most solicitors consider it to be a gift now. Don't think for a second you'll get anywhere near what it cost in the shop though (maybe pay for petrol to Cornwall rather than holiday abroad?!).
It's legally yours to sell. I would also advise checking it out with two or three different jewellers (go to high street jewellers/pawnbrokers not the online gold-buyers as you will get a much better price); also ask how much you would get if you pawned it rather than selling it outright.
My ex and I have been in an off relationship for the past 8 years, the first 2 of which we were engaged. I kept the ring thinking every time we got back together I'd need it again at some point. I never did. Times are hard after Christmas and I'm wondering seeing as though it's not going to be needed would it be ok to sell it?