Your William is an elusive fellow! I searched the Burials in Berkeley (which includes Newport folk) and Chipping Sodbury between 1830 and 1840, but could not find him. In fact I could not find his wife Elizabeth either, and I looked at the PR 3 times to be sure. I wonder whether you got the information about her death from a death cert, as I see she is on FreeBMD. I can only think that although she died in Chipping Sodbury, she was buried elsewhere.
The only Powell burials I found in Berkeley were:
Emily age 10 years and 6 months 29 April 1833
William age 33 31 July 1833, as mentioned before
The only Powell burial in Chipping Sodbury for the same period was:
Horatio Nelson age 7 months 3 March 1833 (I wonder who he was named after, poor little mite???

)
Sorry I haven't been able to help more. If you get any more clues, I'd be happy to follow them up.
Gillg