Tsk, the Matelots brave attempt at sartorial standards for this type of function leave a lot to be desired. You should of course have begun this thread in the Corps forum for the Royals, the only true arbiters of female fashion, to cogitate on.
A ladies suit from the House of Chanel would be most appropriate, dark or Navy Blue would be most suitable.
Shoes and hose are of course up to you but should be dark. A raincoat or overcoat from Aquascutum, or similar reputable clothier should be taken if the forecast is for inclement weather.
Have a gin in the Mess for me.
'Struth Bishop. If you're asking this question, you should go in your ecclesiastical rig, minus medal ribbons. The lady should dress in what you have left or refused to wear. She should also wear a 5cm badge, on her port breast, announcing, that she is attending as your secretary(so no one gives her the grief that you should get).
Hope you enjoy your soiree.