I’m 5’9” and am so fed up with M&S inconsistent sizing. Until recently I was always happy with M&S Per Una Roma fit. I always used to buy longer length but then they suddenly got really long. (I think it must be because those huge platform shoes came back into fashion.). I then got a few pairs of regular and found they fit well. This month I brought home two pairs of regular, only to discover they were too short, so I ordered the same size in long. Well - the thighs were really baggy and the crotch was drooping. Absolutely dreadful! Like you, I too am at a loss as to where to get jeans that fit. (I have favourite Roma bling but have found with the navy and plum ones they are fading with washing.)
I also agree that the quality of fabric in most of the jeans in M&S this year is awful - and no doubt there will be degree courses soon on Jean ‘styles’. The whole ‘skinny’, slim, straight, slim-boot, super-skinny carry-on has me baffled. Just like all the varieties of coffees!