I'm sure you've got better things to do but in case anyone wondered, and apologies for the mise en abyme of threads, I thought I'd update on this.
I returned with the dress - the manager refused a refund in person. I accepted, very unhappily, an identical dress. I still liked the dress very much, the fit was great, but I was deeply unhappy about the quality. Wore it to another wedding this weekend, and wore it the next day with the skirt down, as a summer dress. Decided to reconcile myself to the dress and almost began to feel that I had made a fuss about nothing. Then I hitched the bodice up slightly, and, lo and behold - it fell apart again in the same place. I couldn't believe it.
After an email and two phone calls, the manager has very unhappily agreed to give me a refund. I'm sure she believes I've made this happen on purpose. Thanks to all for your support on this.