My DD has been wearing a 30c, took her to M&S just before Christmas and the lady put her in a 36A.
Now I'm not sure this is the right size and DD won't let me see her in just her bra 
I'm not sure if her dress size is any indication to her back size but she wears a size 8/10, but her breasts are, how can I put this, bloody huge!
She won't let me measure her the way that has been discussed on here ( I changed from a 42b to 38d thank you!).
I'm just not sure an A cup is right for her and I certainly don't think she is 36 around the back, I'm sure she measured at 31 but can't be certain.
We're in Edinburgh so have a decent selection of shops, are debenhams any better than M&S?