On the M&S website is a 'Bra Fitting Tool'. I've no idea if it's a new thing, but filled it in anyway, out of curiousity.
(I wear Bravissimo Satine, 36G, fitted by Bravissimo, fits perfectly (IMHO)).
I fill in all the details, I tell it that no, the back isn't riding up, the cup isn't too small/big, the shoulder straps aren't too tight or too loose. In fact, a perfect fit.
It tells me I should be wearing 38F.