Lisalisa I'm crap at links but could you look in Hobbs or Jigsaw ? Hobbs often has fairly posh looking stuff in interesting colours. jigsaw colours are great too, but perhaps on reflection more "bohemian" than the fitted look you're after.
Definitely look at wrap dresses thoughj; I'm not size 18 but I wore one by Issa (stocked in Selfridges, Harvey Nicks etc and independent dress shops) shortly after the birth of my 4th child, in order to go to a smart function. Very flattering. I then sold it on eBay - described as used of course - once the baby weight had gone. Virtually got my money back. Issa wrap dresses are c.£300 I think, but you could eBay it, or take it to a dress agency near you.