Who does William say is his father on the marriage certificate?
Is William the one who married Isabella CHARLTON in 1842? Is he the one who appears on subsequent censuses born c1819 at Simonburn? If so, a likely baptism at Simonburn on 8 July 1821 to Robert (a mason) and Mary. Does this fit with the info you have?
Possible marriage at Simonburn of Robert ROWELL and Mary HERDMAN on 15 March 1806, and children baptised at Simonburn to Robert (mason) and Mary:
1807 John
1813 Mary Ann
1816 Hannah (privately)
1816 Jane (privately)
1821 William
In 1841 this looks like Robert and Mary at Simonburn (with presumably daughter Mary and her children

)
Robert ROWELL 60 (mason)
Mary 60
Mary 25
John 10
Jane 4
Hannah 8 months
HO107/842/21 pg1
I can't see any other ROWELL's at Simonburn in 1841.
There is a William ROWELL (20, lead ore miner) at Allendale in 1841 (with Elizabeth 55, John 25 and Elizabeth 15) though I'm not sure that this is your William as he appears in subsequent censuses with wife Elizabeth and family at Allendale.