Unsuitable present

I can help you there! DH got a jumper for Christmas from M and S. He has returned it already and been given a credit voucher which is valid for a year and can be used to buy anything they sell. The only problem is that if the item has been reduced in the sale you won’t get the full value.,

Without a gift receipt, I think M&S will exchange it for another size, or a voucher.
If you had the receipt they would give you your (her) money back.
That was their policy a few years ago - it may have changed. Why not take the item back to a store and just ask? Nothing ventured nothing gained (as they say).