It might be worth getting the birth certificate of one of the children b. after 1837, just to get Sarah's maiden name and try and find out where her family were around 1830-1835.
All of the children listed on the IGI seem to be chrs. Mortlake, but those marriage records are on the IGI and Lot doesn't seem to be listed. So next place to try would be the places of origin of Lot and Sarah - according to 1871 census these are Elverston Hants and Wakenham Berks. (possibly 'Elvetham' in Hampshire - not on the IGI - and Wokingham Berkshire - only on IGI up to 1812)
Is Lot's father the Lot Bennett aged 65 in 1841, in Crandall, Hampshire? That would be another place to look. (this place is also not on the IGI except some records for the independant church).
www.a2a.org.uk has some info on Lot in Mortlake, incidentally. Including a note 10 Feb 1838 indicating 'Loat Bennett' should be given a gratuity for detecting a thief in Mortlake Church, and another later that year making 'Lott Bennett' a watchman and signing him and some others in as constables.
http://www.familyresearcher.net/BritishResearch/Hampshire.cfm?letter=Be - Lott Bennett (presumably the father?), married 1795 in Crondall.
