To the most recent I wore a beige/gold floor length dress that had a black belt directly under the bust (that was satin with flowers on and tied behind my back) and a black strip up the middle of the bust (hard to explain), satin again. With black shoes, a black choker and a black bolero. This was in October. No pics though, sorry!
End of season dinner last March I wore a pink halterneck satin dress with a low back that cut off just below the knee and had stiff petticoats underneath the skirt to make it stick out. With the same black shoes IIRC.
TK Maxx is the place to go! Lots of sparkly long and short dresses, max £50 usually!
You can wear anything these days I reckon, as long as it is a dress and a bit evening-y. I wore a terribly flattering long black dress from Jigsaw last year, but wished I'd saved my money and gone to TK Maxx as I actually felt very boring. It is quite hard to be too glam at a hunt ball, so might as well go all out with some sparkle and colour!