As far as I remember Ann Harvey is expensive, mostly older women's stuff and evening dresses etc, if your customers are used to paying £10 for stuff I really wouldn't stock Ann Harvey stuff.
I buy a lot of my clothes from Simply Be, on occasion their stuff can be cheap, they are starting to stock more designer stuff now though and it can be a bit more expensive.
Evans is good, generous sizes and the tops are usually a good length etc, but most towns do have an Evans now. Their younger range Seven or East Coast can be good, but some of their range is a bit naff (chavvy tshirts and velour tracky bottoms etc)
No idea about M&S because I assumed they wouldn't do my size (26)
Newlook are awesome, at least for my age group, and cheap! There are several ebay shops which do bigger fashion clothes, such as:
Big Girls Only
Big Girls Wardrobe
Pretty Big Stuff
Jeanettes Bazaar
Satsoomas-16-Plus
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In any case, well done you for stocking bigger stuff! It's SO DAMN HARD to find shops which stock big stuff, so on nights you find anyone over size 16 wearing the same evans top as you, it's a nightmare! Word spreads quickly so you'll always be in business if you stock plus size clothes! Just get a sign up in your window saying what sizes you go up to.