Update 1 year on.
Magnesium was helpful at first but after a few months migraines became more and more frequent, at least 3 a week, untreatable - nothing touched them.
Stopped everything for while and gradually reintroduced the 400 mg B2,1000 IU D3,800 microgram Folic acid + a few strands of saffron daily but no magnesium.
Migraines are down to about 3 a month and those have been treatable. (The saffron was the recommendation of an acupuncturist but no, the 8 months of acupuncture didn't seem to help - couldn't even reduce a headache present at the time of treatment).
Everyone's different and migraines seem to follow cycles so one can never be sure what is and isn't helping, but on this regime, so far, so good. Possibly magnesium can be overdone, over time.