You are entitled to either a refund, replacement or repair under the sale of goods act 1979 (as amended) if the goods are not fit for their intended purpose or of the expected quality, they must be returned unused within a "reasonable period".
This reasonable period can last up to Six years in the case of certain goods, therefore a pair of jeans such as in your case would be deemed to have been returned well within the reasonable timeframe allowed under the above act.
Insist that you are rejecting the goods under the above act and persevere with your claim for a full refund, if they will not co-operate.... seek the involvement of your local trading standards dept.
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Incidentally; firms like Marks & Spencer and BHS amongst other leading chains, reject clothing that has stitching problems however minor ...and subsequently sell on the sub-standard products as, "unfit" to market traders as "seconds" after first removing any labels to indicate where the clothing originated from.