You're welcome dangermouse, I hope it was of some use. I think if we know the reason why things are happening, those things are easier to handle.
I've had the conversations re caffeine with some top dog mig specialists. It helps some people whilst it's a trigger for others. Some migraineurs find that if they drink a coffee even after the migraine starts it helps reduce the intensity: it would seem it unites with the pain meds in some way. I've never found that food has been a trigger but I don't drink any alcohol and wouldn't drink high caffeine drinks. I feel so sick when in migraine that I couldn't stomach coffee but sometimes drink one or two a day, usually decaf.
The glucose can be absorbed in the stomach and takes the meds with it apparently. The stomach is in stasis in migraine and stops transit which prevents meds being absorbed. It may be worth trying when you're struck down with an attack of gastric migraine, especially if you can take aspirin which seems to work well on migraine. Worth a try, it may abort an attack earlier. Boy oh boy, it's finding a way to manage all this stuff!
Do read that link to The International Classification of Headache Disorders on the Migraine Trust link I posted. It covers all types of migraine, and there are many, and also all types of headache. I seem to remember gastric migraines are discussed, you may get some hot tips!
Good luck dangermouse, it's amazing you even got a diagnosis for this, they are so tricky to diagnose. x
