I hope your GP has some suggestions for you. It sounds like you're really suffering.
My suggestions for pain relief would be most of the techniques you've already tried (from least to most drastic): hot water bottles, peppermint tea, hot baths (none of these seem to do anything for severe pain, I realize); anti-spasmodics and/or anti-diarrheal drugs; selective serotonin reuptake inhibitors (SSRIs); codeine; narcotics. The narcotics are the worst idea, since they tend to give people pretty heavy gut reactions like nausea, vomiting, constipation, etc. What are you taking right now for pain? Do you know the active ingredient?
Do you have someone helping you out in person? A family member or friend? I think you need some personal contact to help you deal with this...someone to go to doctor's appointments with you, be there to support you. That's a lot to ask for in someone, but if you do have that support, you're definitely ahead of the game.
Have you researched biofeedback or hypnosis, by any chance?
Let us know how your GP appointment goes. Best of luck.