hey aquablue, i'm sorry to see you find yourself in this situation but there is really and truly only one option here and that is to sell the property. I agree with other post's that the main thing is not to panic. however, it's always best face these things head on rather than dragging the whole process, stressing yourself out at the same time.
you need to appoint your own solicitor (assuming you're not legally seperated), then go and take advise from him/her. i would equally be recommending that you and your ex go and meet with the bank and point out exactly whats going on. an agreement will then need to be reached in relation to the shortfall.
Again, your solicitor can do all this for you, that is, if you feel you didn't have enough knowledge on the subject.