It sounds like you've measured properly (under bust tight and across bust loose) so let's work on the measurements 27" and 33".
You're between band sizes 26 and 28. You should try a 28DD and a 28E and ideally a 26F and a 26E. However, 26 bands are not readily available although they can be made to measure by a company called Ewa Michalak in Poland. There is another company who are now making 26 bands but I can't remember the name. Another bra geek might know.
If you can get the right cup in a 28 you could then adjust the back by removing some material (lots of info online about how to do this) or use a Rixie clip (eBay).
You can also upload your details to this website for the future gain of many small backed ladies!