A long time ago, well lets be honest, back in the 1960s, I worked for a large firm of Management Consultants and as part of my work saw all the business magazines. Computers were just coming in and M&S were in the vanguard. The magazines would be full of articles saying how wonderful M&S stock control was, how shops were kept fully stocked with all the most popular styles/sizes etc etc. In my lunchtime I would go to M&S to find that what ever I wanted they didnt have it in my size/colour I wanted/length because it was so popular/not much in demand etc and that has never changed. On those rare occasions when I go into M&S if I see anything I like, and that is even rarer, I can bet my bottom dollar that it is not in stock in my size/preferred colour/length and I am 5ft 4 and a standard size 14.
I have also always found that their styles are aimed at women at least 20 years older than me - and at nearly 70 I still find that. I visited my local M&S recently and the majority of the customers looked considerably older than I am.