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I've never had any luck with M&S tights - they're always quite Baggy, but never long enough.
And don't get me started on Next!! I bought a gorgeous pair there once, in a L, to find out that the body part was just as long as the leg part, and the worst length I have ever known - I stretched them so hard and they still came to mid thigh. I gave them to my really short assistant, and she gave them back because the legs were too short.