More than 15 years after leaving higher education (I'm 40) I finally have tickets for a summer ball at my Oxford College 
I've never ever been to a summer ball and am hugely excited. There's about 6 weeks to go and I suddenly realised I have nothing to wear and no clue about how to get this sorted out
Your mission, should you choose to accept it, is to find:
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A dress which will be stunning and sparkly and properly 'ball' like and which also won't make me look like mutton-dressed-as-lamb amongst all the scarily young students. I'm clearly not going to wear it again (or at least not on a regular basis), so don't want to spend a fortune, but of course don't want to look too cheap either.
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Shoes which aren't too high as I have no balance issues with height and crap knees. Although very blocky things might be ok.
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A bag
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Maybe a second pair of flattish shoes. And maybe new underwear too...? 
Budget: about £100-150 on the dress; £70-80 on shoes (more likely to wear them again); £30-50 on bag.
Vital stats: I'm 40, 5'7", size 10/12 (smallish waist, 32E bust - so prefer not to go braless and struggle with strapless). Feet size 5.5 (39). London based, so can theoretically get to shops to try things on with patience, commitment and a following wind.
Can anyone wave a magic wand and help me out please?