I went to Bravissimo today (nearly 2 hours away, so we made a day of it) and thanks to the helpful ladies on here, I'd fitted myself last week with a bra that really wasn't too far wrong. The shop was absolutely brilliant and the staff couldn't do enough. I now have another new white one, a black one, and a pretty pink one. 
I can't afford a whole new wardrobe, and am probably going to take a while to lose my excess stone or two, but this is the first step along the road of doing something for myself for a change, and feeling better about my appearance.
I also got some good advice about how to fit my daughters when the time comes.
Thank you, intervention people. I am your newest convert! (although perhaps not that new, as I started looking into sizing issues when M&S tried to fit me with a 40" back when I was halfway through my first pregnancy... I'm a size 12) 