Thank you very much, you've confirmed Adam was Richard and Martha's son. The burial of Daniel would fit if he was baptised at 4 months old, which is quite possible.
I have found baptisms of three Johns, the 1827 baptism is the middle one, the last of the Johns was baptised on 8 May 1831, so middle John must have died before then. As it looks as though Daniel was born Dec 1829, I would think middle John was still alive then, otherwise Daniel would probably have been called John.
The John who died in Wantage workhouse would have been born about 1824, so he doesn't look like he's the right one.
Since posting my original message, I have been looking at some local papers. I knew Richard had been transported for life, but now I have found a reference to Jesse in reports on the Chilton Fires.
Devizes and Wilts gazette 2 mar 1843
"Richard Wallis, Jesse Wallis (his son), Richard Wallis, Joseph Annette (sic), Joseph Blisset and James Wilson" all transported for life except Jesse who was transported for 15 years.
I don't know whether repeating "Richard Wallis" was an error or there were two Richards. I haven't come across another one. I'm related to all except James Wilson!
So two sons accounted for, still Adam and John to find.