Celia, I am so glad your dress was ready for this event! You look beautiful, and the clutch was a perfect choice.
I understand absolutely what you mean about the "ladylike" feel of this dress and it is something I struggled with, too—to the point where i seriously questioned buying it and almost took it back. But my husband really loved it, so I cut the tags and it's at the dressmaker for its alterations now.
Before I decided to keep it I told myself I MUST find a way to make it feel more casual. And I did it! First I tried with my denim jackets, but strangely, they did not seem to work particularly well. One was too faded and just didn't seem to work with the texture of the dress, and the other was a bit long and got bumped into the self belt in an annoying way. However, I managed to fix that issue by finding another way to secure the belt (with a pin rather than tying in a knot) so in a pinch I can wear it with denim as a dress down.
I liked it better with a long waterfall cardi in navy (however, this did still feel a bit "ladylike") and with a short sleeved coat sweater in black (less ladylike). When it comes back I will try to get pics for you if you like. The coat sweater is similar to Claire's idea of a waistcoat because that is more or less the effect of this particular coat sweater; it does up at the waist but falls open below, and it has a wide collar.
Boots and booties were my best tools. I found some textured tights at the Gap (similar to this: http://www.gap.com/browse/prod.....tights.jsp (except mine are a lighter colour, more like dusty mauve) and I put those on with black booties; it really seemed to make the dress feel much more casual.
Tall boots in grey or black would also work.
Good luck, oh Dress Twin!!
BTW, my seamstress was head over heels about this dress; she thought it was just divine! The colours, the fabric, everything. So we're clearly on to a good thing here.