Thanks Kath,and I have to say that I have checked the Baptism for Hannah and other siblings,at the Chester Record Office last Thursday.One thing is for sure there are no earlier Baptisms for any children of the couple,Hannah being Baptised on the 6th of August 1820,and given the ages of Joseph and Elizabeth in the 1851 Census,this would more likely to be the case.
As far as the Marriage is concerned,I am being perhaps entirely speculative,for the Marriage in March 1820 at St Oswald's is taken from a submitted entry on the IGI.Whilst there are two possible Baptisms for an Elizabeth Johnson in the Tarvin Parish around 1800,it does seem a mystery as to why she would marry at St Oswald's which is about seven miles away.
Regards
William Russell Jones
Cefn Mawr
Wrexham.