I gave up on maternity wear in the end as it was too pricey and didn't fit all that great, sometimes felt the clothes were too tight, so instead I've been getting larger/oversized tops, jogging bottoms, nighties etc in places like Asda and Matalan.
I can really recommend Matalan for nighties with the buttons down the front, I found a fab grey star-patterned nighty for only £5 and have been getting some fab tshirt-style tops and 3/4 length sleeve cardi tops from Asda for about £10 each. I also picked up a couple of pairs of comfy black jogging bottoms for £3 in Asda, don't be put off by the price being so cheap, I tried them on (2 sizes up from usual size) and they are so comfy over my bump and will do the job nicely. I also got all my big pants from Asda, same cost if not less than disposable ones, in black and white colours, black to hide leaks, white means I could bleach them or just throw away they were so cheap! I also got a couple of big baggy mens tshirts from there too for a few quid each, perfect for lounging around the house or hospital. You don't have to spend a fortune.