Thanks for the advice, ladies. Yes, I did put the bra on backwards (the 38F) and actually went as far as buying a few as they seemed to fit so well. It was only after I really started moving about in them that the sides started folding out and up.
I measure a 40 around my chest without adding inches on. I can just about get the 38 done up (it's tight) so I don't think I can go down to a 36. The thing is, when I tried the G cup on in the 40 and the 38 back, even after scooping there just isnt enough boob to fill the cup. It is so empty at the top that I could fit my fist in with space to spare. I am at a complete loss!