Have you tried the www.yoursclothing.co.uk website? They do sizes 16-32.

I recently discovered them on Ideal World (shopping channel). I was going to buy from them on their website, but checked for a local store and they have one in the next town so I visited the shop instead. I think they have some fashionable stuff for larger ladies, I am about a size 22-24 and find it really hard to buy clothes at the moment.