OK, I am 18-20 (was 16-18 before DS2 hmmmm) and generally am considered to be well dressed by my friends and colleagues. I buy from H and M, am quite norktastic (36HH) but thier L/XL can fit well often especially in lycra-ed tops, which are plentiful and cheap there! Per Una goes to 18 and 20 but you do have to have a good search through the stuff there. I'm not a huge fan of Next, I think I am but am hard pushed to think of anything I own from there that I wear lots apart from a couple of dresses I got in a sale a couple of years ago, and thier linen trousers which are fabulous. I think Evans can have gems if you explore, and I do like thier shoes. Dorothy Perkins, Wallis, Principles do 18/20s (DP goes to a 22 I think???) Monsoon you've mentioned of course. I have just bought a dress for my sister's wedding from Simply Be and I really like it, also some shoes from there which are yummy. Even Primark can come up trumps sometimes, I have a couple of linen skirts and linen trousers that swim on me, and I love them. New Look as mentioned above and sometimes Tesco/Asda.
I think the key is to search and keep aware, you can send catalogue stuff back, I have a lovely relationship with my courier lady, she's really nice.
Accessories also are very helpful!
I think all of us lovely ladies are aware that it would be helpful to lose weight, you'll probably find most of us have, and have slipped off the wagon in our time. In the meantime, whilst achieving this goal, it is in our own best interests to do the best with we can with ourselves, and indeed we owe it to the poor unfortunates who have to look at us, non? Or, perhaps people prefer to see larger women in jogging bottoms and marquees....