Have you found them in any census at all? I've just had a quick look and cant find them. I also looked for william's birth on familysearch.org and there wasnt a single Malpass born in clay hills on that site, which is usually helpful.
I did find a William Malpass born in 1813 in Bitterley, Shropshire, he had a brother called John, born the same place. They also had many siblings. There parents were Benjamin and Elizabeth.
Hope this is some help.