The Thos I am looking for was listed as 40 in 1861, making his DOB 1821.
But in 1851 his age is given as 36, making his date of birth 1815!
Horsley was a Township of Ovingham Parish.
There is a Thomas Lathan baptised at Ovingham in 1818. Son of Michael and Catherine.
Michael & Catherine (entered as Leathen) were living in Horsley in 1841, and Michael was a Mason, as was your Thomas. (HO 107 / 841 / Book 7 / Folio 5 / Page 7 )