So far I have the following names with parents John and Sarah in the St James parish.

Joseph 1817,baptism 01.06.1817
Mary 1818,baptism 22.03.1818 (believed to be your Mary)
John 1818,baptism 01.06.1818
Ann 1834,baptism 08.06.1834
Eleanor Elizabeth 1834,baptims 08.06.1834

There is no joy on the 1841 census at the moment for any of the above, excluding Mary.
I have got burials of a John Hill aged 60 15.11.1831 and Sarah Hill aged 54 08.02.1837 but neither of these can at this moment be verified as part of the family but they are in the St James parish, but they could have died elsewhere.
I am hoping the transcript of the 1/4 sessions may throw up some light of where the family used to live.

That is all at the moment.