Following on from the concern about the Thomas & Margaret Bartley found in 1851, it seems to me in fact that they can't be yours because Margaret Bartley nee Plumb had remarried to a Thomas Kent on 1 October 1837 and died in 1844.
The 1837 remarriage shows Margaret as a 23 yr old widow, her father named as John Plumb, a plane maker. New husband Thomas Kent was a 27 yr old mariner; both parties gave their address as Cumberland Street.
Margaret Kent, 29, of 38 Cumberland St was buried on 16th February 1844 (Holy Trinity, Liverpool).
She appears to have had a daughter Margaret in 1840 who (like half-brother Thomas Bartley) appears with the Plumb grandparents in 1851. She is likely to be the Margaret Kent who, together with Isaac Plumb, witnessed the Thomas & Esther marriage in 1858. She herself married Edwin Brown on 12 April 1861, giving her address as Cumberland St and naming her father as Thomas William Kent (deceased), Master Mariner.