Do NOT go to M&S - they systematically misfit women I am afraid. They basically us an old fashioned method where they add 4 or 5" to your back measurement resulting in a bra which is far too big in the back, too small in the cup and doesn't support properly. Sadly a huge number of shops do the same
Best bet on the high street is Bravissimo or John Lewis
Have a look here for fitting advice if you want to get an idea of what your correct size is before going.