Another recommendation for lessbounce.com - the staff are very helpful and they have an excellent range of styles and sizes. It is a bit of a pain that its mail order, even though they do free returns its still the delay in getting the bra, trying it, deciding its no good, sending back, getting another one etc.
Agree, Sweaty Bety don't cater for smaller women and M&S are a bit hit and miss. You need to try on every bra as there appears to be a complete lack of continuity between designs re sizing.
I have the same issue with M&S sports bras, you really do have to try on a range of sizes in every style of sports bra you want to find the correct fit.
I would really like it if running shops branched out into supplying a good range of sports bras, with a treadmill test so I could check how it felt while running & if it was offering enough support. You don't get the same effect by jumping up and down in the changing rooms at M&S! 