First check that you've actually complied with the washing instructions. I've never seen a pair of jeans labelled 'dry clean only' but, if they are, you haven't got a leg to stand on. Similarly, you can't make a claim if you've washed the jeans at too high a temperature or if you've tumble-dried them when there's a symbol on the label which shows a circle, inside a square, with a cross on top of it.
If you've complied with the washing (and drying) instructions, your rights to a refund are 'borderline' in law. You only have the right to a refund (rather than to an exchange or repair) up to the point when you're deemed to have legally 'accepted' the goods. The length of the pre-acceptance period isn't defined in law but basically it just gives you enough time to get goods home and check that they're of acceptable quality. My opinion is that putting the jeans through their first wash should be regarded as the final part of checking that they meet the required standards. i.e. I'm of the opinion that you've not legally 'accepted' the goods and therefore you retain the right to a full refund.
However, the shop might argue that you've gone beyond the 'pre-acceptance period'. If so, you're only entitled to a replacement or repair. (Since a 'repair' is clearly impractical, that means that you'd be entitled to exchange the jeans).
Either way (assuming that you correctly followed the washing instructions), the shop is definitely obliged to offer a remedy to the problem. They definitely can't just say 'Nothing to do with us'.
Chris