Hi Oxo,
Firstly they look at the size you are wearing. They say you shouldn't be able to pull it at all at the back. That's why I went down to a 32 because the back was too big. The back should sit much lower down your back than you anticipate and should be straight across without any pull on it.
The front of the bra where the two halves meet (can't describe it any better than this!) should sit flat against the breastbone, again there shouldn't be any room to get fingers inbetween etc.
The whole of the breast should be covered by the cup (unless a plunge style I presume from their catalogue).
The fitter starts with the back size first and then they increase the cup size until your whole boob is covered.
I went for a smooth bra for wearing under t-shirts, it looks massive but is very comfortable and you don't notice it. It's not a bra to wear on your first date thats for sure but they do have some very pretty ones.
I guess my boobs kind of feel hiked up to where they should be. The look more spread out rather than pushed together.
I haven't bought anymore, as I am trying to lose some weight, although my dh is keen for me to get the plunge bikini and shorts set for our holidays in a couple of weeks time!
HTH