Ah, we've all done it! A lot of the indexes on FamilySearch are from BTs, and there are often gaps in them.
Hopefully you can track down the baptism of William at the archives (don't know why we can't find him on the spreadsheet). Hopefully Relph will be correct, and the surname may look something more like Gilbank/s, etc.
There should be an occupation for John as well, which you can cross reference with your 1816 baptism (and with Elizabeth's baptism as well, hopefully)
Good luck!