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Did your fiance's mother buy it from a shop or was she distance-sold it by telephone/mail order/internet etc?
If she bought it from the shop you will be bound by their refund policy unless you can show that it wasn't incorporated in to the contract (ie. not brought to your fiance's mother's attention) or that the refund term itself is unfair under the Unfair Terms in Consumer Contracts Regulations 2000. Most high-street retailers print this on their receipts, or otherwise mark them very clearly but check in your case anyway.
The OFT's website is pretty user friendly and you should check what term relating to refunds:
3 weeks sounds like you should action the above very quickly though and certainly attempt to return the dress as a matter of urgency.
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