I attended the National Migraine Centre last week, feeling fairly sure that I would be told that I was taking too much medication - namely sumatriptan (Imigran). I was right.
It seems that the best course of action is to go "cold turkey" - just cut out all medication - for a week. By the end of the week, I am told that I can expect a fairly dramatic improvement in the frequency and intensity of my headaches. Wish me luck!
If I don't manage to "stay clean" for a week, there are methods that could help me to cope more easily with the withdrawal symptoms. I won't mention them here as I think they are best discussed with a specialist medical practitioner.