Monsoon is expensive but does gorgeous, classy clothes. I have a lovely ball gown from there. It is straps as opposed to sleeves but I have a blue satin shawl I wear with it.
If it is a bit of a summer party, what a bout a sexy wrap dress? Mine is just below the knee, and although it has a plunging neckline, I wear a black strappy top underneath to hide the cleavage. Its great because the colours make it look like part of the dress.