I have always been small breasted, and currently have a collection of VERY old M&S bras, all in 34A.
I need some new ones, so went to M&S and tried a whole load on, in 34A. In most of them I have a gap at the top between the bra and my breast. A quick bit on internet research tells me that means I need to go down a cup size. So I try 34AA and 36AA. 34 band size seems pretty comfy but none of the cups seem to fit right.
Now, I've just been on the Boob or Bust website as their size calculator was recommended to me, and I came out at a 28D 
Does anyone else have major issues finding well fitting bras for small breasts?!
I'm feeling extra miserable about it at the moment as I recently stopped breastfeeding and had been enjoying having temporary B cups thanks to the milk 