I've just sent two pairs of M&S trousers back because of their erratic sizing.
For the past few years I have always worn size 16 in long length from them, but recently I haven't been able to find a pair that fits. I thought I'd cracked the problem by ordering two pairs in different sizes and leg lengths! When they arrived the size 16 was too tight, the size 18 was miles too big and baggy. The regular length which according to their website is 31" was too short (when I measured them they were 29") and the long length were 33" and too long.
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My friend bought a pair of black trousers in store just before this lockdown and liked them so much that she decided to buy another pair in a different colour, but same size and length. They bear no resemblance to her original pair!