What an excellent thread - I certainly hope M and S take note of the comments. Did you know that if you post a comment on their website they vet it before it appears? I used to always leave a comment then for some reason they started rejecting my posts so I gave up. I think they don't like it if you make a negative comment.
I agree with all the above, M and S have really gone down the pan. As others have said, they need to clarify their target market. Under 40s will not buy from M and S except maybe for their children, so concentrate on the older age groups, listen to our comments, give us what we want - and hey presto! your sales will go up.
In answer to the OP:
Get your sizing right as at the moment it is a disgrace, especially in trousers. I used to be able to pick up a medium length trouser in my size and know it would fit without trying it on but now I have to try on several pairs in different sizes, so I don't bother.
Sack the celebrities - we don't need them, we don't care about them, they don't shop at M and S anyway.
Put the larger bras at the top of the display where they are easier to sort through. Those with small busts can more easily reach lower down. Get your assistants to check the displays regularly as the bras are always in a muddle. Many's the time I've walked out in disgust when I can only find size 36 and 38 where the 40s should be.
Bring back v-neck, cap-sleeved t-shirts in nice colours - get rid of bright turquoise and shocking pink - and put a bit of shape in the styling and skip the silly chest pockets. Also, where are the 3/4 sleeves?
Ditch the acrylic and polyester and give us some nice cotton dresses (with pockets) for the summer along with cotton cardis and jumpers. Polyester is so not wanted. We'll pay a bit more for natural fibres if they're good.
We need some pull-on bootleg jeans - skinny ones are not for everybody.
I could go on and on......just hope GN can pass this thread on to M and S. As someone says above, market research is expensive but we are here giving it to them for free! M and S TAKE NOTE.