I'm in my mid/late-30s, and weirdly I used to shop in M&S a fair bit in the 90s and early 2000s when I was a teenager and in my early twenties - they still did some basics and workwear which were pretty wearable even if you were young. And decent shoes. I had some plain skirts and blouses, my first suit, basic cardigans, I bought all of my shoes there, all of my underwear. Now? The last time I bought clothes from them was in about 2005 when you could still get cheap round-neck cardigans that washed decently.
It's not that I am massively better off now (at least not after DC 
) but I genuinely cannot find any decent basics in there now. I discovered I was buying the wrong underwear sizes (I'm a small back and large cup size), so moved to Miss Mandalay, Mimi Holliday and similar lingerie companies which cater for that sizing. That market expanded massively around 2010, but M&S didn't bother to keep up and I now can't find sizes that work for me there.
For basics I now stick to Gap, which can be hit and miss some seasons and goodness knows I would like their designers to go back to basics a bit more too, but they can still be relied on to stock basic black cardies and jumpers, basic t-shirts, comfy modal nightwear and minimalist leather bags. More upmarket things, I go to Jigsaw or similar. Just can't find any decent basics in M&S. Don't even get my shoes there any more as I just don't bother to go in.
The only thing I do buy there for myself now is the magicwear opaque tights. I get an online order of 10 of them every couple of years when the last ones wear out.
The baby range is actually great - fab plain and attractive good quality cotton sleep suits that fit and wash perfectly. Good sizing for babies. Nice cotton basics. BUT - the girls' ranges above 24 months? DREADFUL. Some clothes seem designed to make your pre-schooler look like a mini-hooker - sequin hot pants, black tops with lipstick prints on the front, yuck. Others which look vaguely decent (Boden copies of corduroy dresses etc.) sell out immediately and never come back into stock despite clearly being the only things that are actually selling. It's not hard to design kids clothes that sell. Your target market for kids clothes do not shop at Asda, so just copy Boden, JoJo and John Lewis and price it lower!
In fact, all M&S need to do is, as everyone on the thread has said, is dramatically reduce their range overall, produce a smaller range of good-quality basics in natural fibres, and look to Boden, Joules, Seasalt, Gap and Muji for inspiration.
Nice plain merino, cashmere and cotton cardies sized well, so that you don't swim in them, in a range of neutral tones and jewel colours! Good quality jersey wrap dresses in understated prints! Basic lined black skirts for the office! Plain elegant modal camisoles and tops. White t-shirts. Plain black and coloured coats that actually fit decently. Girls' clothes in block colours and good fabrics that aren't covered with Peppa Pig. Come on, M&S; it isn't hard! I can't believe I wore your clothes to work when I was 21 and a size 8; but now at 37 can't find a single thing that isn't far too frumpy to wear.