As suggested by Trish: pretty shoes. I'm loving the peep toe and the buckle.
I was considering the 'going out' bags I already had and came to the conclusion that most of them should probably go into the dressing up box. There's nothing wrong with them except that they were fashionable about 10 years ago and were last used about 8 years ago. So... a pretty little up-to-the-minute clutch purse.
And I love the material lining. Pretty, pretty, pretty.
I vetoed the suggestions on dresses and skirts. I'm just not comfortable in them and, while I will be a little outside my comfort zone simply by dressing up I don't want to be so far out of it that I feel like a freak. Even if I looked okay I'd feel weird and that would just put a downer on the whole night. I found a pair of black pants that are just gorgeous - they're just regular black pants, pretty much the same as every other pair I already own except that they fit beautifully - skimming my butt and thighs rather than sticking to the lumpy bits - and just sit really nicely. I tried really hard to buy something sparkly. Really hard. But nothing fitted / sat nicely / stayed where it was supposed it / covered the bits that really should be covered / cost less than my fortnightly mortgage payment. Then I found a plain black T with a lovely ruffle. Not too ruffly that it messed with my head but big enough to add some interest to the top. The t sat well and looked good too. And to go over it a 3/4 sleeve, long-line cardy with ruffly bits too. The photos don't do it justice - it looks better on a body than it does on a hanger. Promise.
I'm going to wear my big gold hoop earrings and check out what I have bead/necklace wise, and make a raid on a girl-friends jewellery box if I need to, and that's what I'm wearing! It should be dressy enough to look the part yet still close enough to my comfort zone that I can enjoy the evening.
Any positve feedback would be most welcome, criticism not so much - the credit card is smoking and I don't have time to exchange anything anyway. But again, thank you. Your advice really helped me out while I was looking for ideas and trying stuff on. You guys are the best.