Good spot Selina! So the 1841 says that Rachel was born in Hunts, although that part of the 1841 is often inaccurate. The youngest Overstall child was only 3 days old. Possibly Rachel was some kind of nurse.
Cutting through all the extraneous stuff on naming patterns, the tradition was that the first son was named after his paternal grandfather, the second his maternal grandfather, and the third his father.
Dave - have you checked the baptisms of the various children with Sawtry parish register, or just taken them from other peoples' trees? There are two parishes which make up Sawtry - St Andrew and All Saints. St Andrew has been transcribed by Hunts FHS and is available on CDrom or fiche. All Saints is only available at Hunts Records Office.
The basic problem with this family is that there are very few Levis in Hunts, which leads me to think that he was from elsewhere, and probably married elsewhere too, possibly nearby Northants.
The NBI, which admittedly is incomplete (eg All Saints burials only start at 1800), only shows 5 Levis burials - 1625 in Easton, then the next was John son of John age 1+ on 20 Sept 1815 at Sawtry St Andrew, John aged 41 in 1825, Cathearine aged 11 on 22 Sept 1828 at Sawtry All Saints, and John Levice age 32 on 29 March 1855 at Ramsey. Sawtry All Saints
There are no Levis baptisms in Hunts on either the IGI or BVRI before 1873, although coverage for Hunts is pretty low. But the Marriage Index is complete 1701 - 1837 and there are but two Levis marriages - Rachel to Robert Wright and Charles to Lydia Stapleton in 1837, both at Sawtry. So John and Rachel married elsewhere.
David