Thanks for your replies!
NurseJen1012: I went back to the doctor last week and he suggested trying Inderal, so I've been taking that since Saturday. I've had 3 quite bad migraines since then, and I'm tempted to give up taking the tablets, but I guess I have to give them a chance to work... It's so good to hear that it's helped you!
BrentD: I can't work out what my triggers are. There are certain things like not sleeping enough, getting stressed, relaxing (I can't win there), alcohol, too much exercise, not enough exercise etc. But often I get migraines when none of these triggers are present - sometimes even just thinking to myself, 'ah, I haven't had a migraine for a couple of days' can set one off. My diet's healthy - I don't eat chocolate and very rarely eat things like sweets, biscuits or cake... So I'm struggling a bit to find out the cause!