I found Next fairly generous in sizes - I am normally a 20 in jeans and their mat jeans in a 20 fall down on me until about the last 5-7 weeks!
George at Asda I got some fab skirts, and maxi dress from and some fab nursing tops also over the bump leggings. Also got some linen trousers I had made into 3/4 length ones for the summer (had a Sept baby last time)
Asos also do a fab plus size range of mat wear.
I bought a load of 3/4 length leggings from ebay to wear under skirts/dresses etc to avoid the chub rub! I avoid wearing skirts in the summer normally but couldn't cope with tights and trousers where a challenge to find. Also find mat tights a huge rip off!
Watch out with H&M some stuff was fine others a bit erratic in sizing.
Drove me mad that 'maternity only' clothing shops (I am looking at you mothercare, jojo, mamas and papas) did nothing past an 18!
My cup size stayed the same and I just used extenders the whole time. M&S pack of 3 for £5 a lot cheaper than changing bras if the cups still suit!
I also managed to get away with a lot of my normal tops which are fairly spacious using bump bands. www.bumpsmaternity.com/White-Belly-Band-for-Maternity-%28Ruched%29/121.htm
Also found these really useful for nursing in post birth as it helped cover bits up in the early days.
I spent a lot of time getting frustrated last time about it.