try South - which I think is Littlewoods online? I brought a pair of size 18 trousers on ebay and they are way too big on the tightest fit.
I also have some Dorothy Perkins stuff but not overly impressed with it - quite small sizing (if you are ordering online) and the 'maternity' tops come up VERY short (ie unwearable after about 20 weeks!!! ) What is the point of that?????
If you are splashing out Isabella Oliver go up to a 20 & are lovely.
I second Isabella Oliver - I am a size 20 and their size 6 is perfect on me. The clothes are all in plain, soft jersey type material and will last you all the way through your pregnancy. I imagine you could flog them on eBay afterwards as well. I mainly live in the smooth line trousers and scoop neck tops
Also try Evans for tops/tunics - I just buy normal tops size 22 and they are fine.
Next is also fab and go up to size 22.
Dorothy Perkins does come up small like FindingAManger says and also I have found to be very poor quality.
M&S also do some bits in a size 20 - I have just got some fab jersey trousers from there in navy (not as good as IO but 1/2 the price and IO 'don't do navy' lol).
ooh I have had the same problem nothing fit me properly. In the end I got a bump band and to my shame I wear it every day.
It's brilliant when I was newly preggo and bloated I wore normal trousers with buttons open and band over it. And now I'm at the finish line I wear it with maternity trousers to keep them up and with normal shirts to bridge the gap when they're too short. I love it.