Meggymoo, whatever size I am, I have problems finding bras. When I'm slim, I'm a 34F, currently I'm a 38G, down from 44JJ as I have lost a lot of weight.
I went to Bravissimo twice and another specialist shop to be measured, got 2 bras on each occasion (they seemed fine when they were fitted), but after about an hour of wearing them I was in agony, so took them back each time.
Being sure of my correct size, I went off to M&S and spent an afternoon trying on bras (you do need to be on your own without kids!). The best range for me is the T Shirt bra range - very comfy, supportive and cut very well for £20 IMO.