simplybe.com do maternity clothes, there's lots on ebay too. Have you tried new look? Not sure what size their maternity range goes up to. Tbh I lived in smock type/stretchy tops, and only needed different trousers at the end (I just wore stretchy tracksuit bottoms for the last month or so)
Actually Dp do most things up to a size 22. http://www.dorothyperkins.com/webapp/wcs/stores/servlet/ProductDisplay?beginIndex=0&viewAllFlag =&catalogId=20552&storeId=12552&categoryId=111569&parentcategoryrn=111560&productId=882795&langId= -1
M&S do different length trousers online. Simply Be trousers are naff I found but I ordered a few tops a bit bigger in size and they have done me - but I am tall and the tops were pretty long.Good luck its a nightmare - when you do find something buy double !
(I was a size 18 when I got pregnant with DS1. And working in the City. Could I find any dressy pregnancy clothes? Could I buggery ... but then I had a hard time finding nice dressy clothes when not pregnant, what with the whole 'fat people have no money' rule.)
New Look mat jean in 18 are generous. Next are only good for tops if your boobs aren't huge. Their mat trousers work on the assumption you are growing a baby not only in your belly but on each arse cheek too. I bought long vests in Evans and wore with cardis. I am short, fat, huge norks, but skinny from thighs down - I look like a jacket potato on cocktail sticks. Not a good look even when not PG!