I think I have finally worked out my correct size. I "measured" 36A at my first fitting and have been moved down the band sizes gradually since.
But, its really hard to buy 30D, unless you want plain black or white.
I don't want anything fancy, prefer a lightly padded tshirt type bra that won't show under clothes, but would like interesting colours and if there are matching or co ordinating knickers and a pretty strap, even better