I ended up wearing the dress involved in this thread to a wedding. The central bust-line seam split within two hours. It's probably repairable but I don't feel that happy about keeping a dress that started to fall apart so quickly. Despite my temporarily ample norkage, the fit was fine.
I left it at the shop with the receipt and a nice note saying I would like a refund.
Just received an answerphone message from the owner (small boutique) offering me a swap with another dress, or a credit note. Do I have to accept or am I entitled to a refund?