Not sure what bra size I am?

Go to your mall and go to Victoria Secret they will measure you for free. They are very accurate and specializes in bra and such things. Mark and Spencers/ John Lewis are department stores so the person was probably wrong. Then with La senza you have to remember that sometimes stores that sell der own bras/undwears have their own sizing charts. Maybe the bra run smaller and u need to get a bigger size. If u have red marks around the stomach and the arms but the cup fits fine you need a bigger band size you might want try a 34 DD in the La senza. Good luck