hi girls thanks for all your responce!! 
Macom- I ordered a Medium as i was a 34 back pre op… fit fine. it stretches to what size you are so you; be fine with whatever back size u are…
With the cheap ones in tesco or asda etc i dont know about sizes as i bought 2 MaComs as i heard they were so good, and they are!
i have a black and white one.
I measure a 32FF/G now!!
and i think there still growing lol xxx