Hi there , I am decended from George Seath and Margaret Beveridges son, John (Born 22nd July 1843).
Georges parents were indeed John Seath and Mary Ann Simpson.John Seath died prior to 1851 as Mary was noted as a widow in that census but was alive at the time of the 1841 census.It also looks like Mary died between 1851 and 1854 as there is no record of her death in the official registers from 1855 onwards (bad timing from both of them).
Georges son Andrew married a Margaret Turnbull Black and died young but I have a newspaper clipping of an interview with Margaret dated 1913 and, although married a second time, talks about her father in law being aboard the Victory at Trafalgar.I dont know if this was her first father in law (George) or her second.I still have to find this our.Could explain why he makes no appearance in the records until 1819 when his first child was born (off fighting the French I guess).
John was born in Largo on the date you say and as he married Amry Ann Simpson, his sister Anne was marrying Marys brother Andrew (complicates the tree somewhat).
All other dates and names are those I have on my tree.