If you are careful about fabric Evans is much better than it used to be - you may just need to avoid the their habit of breaking into glitter at inappropriate moments!! Ann Harvey are pretty good at smart clothes and have a nice online site but if you can get to a store they will carry a much larger range including sale items. Have a look at
Artigiano and Spirito Di Artigiano as they go up to a size 28 I think.
Elvi used to be quite good but I'm not sure if they are still around on the High St...
Gray & Osbourne mail order carry brands with sizes up to 26. Some of these brands crop up in larger sizes at TK Maxx if you have one near you and/or can be bothered to rummage!
Good luck :)