Try Uniqlo Ultra Stretch instead!
I lived in my M&S ones last autumn/winter and was out looking for replacements for this year as I've dropped a dress size and the Uniqlo ones kick the arse of the M&S ones. They're thicker material, softer, better colours (none of that not-quite-right blue that M&S denim always comes in, and best of all are tailored to length free of charge on the day you buy them. Honestly, they're so much better quality I felt I had to share. Totally worth the whole 7 extra pounds they cost.