Re: Thomas Smith of Northamptonshire, England
There's only one way to confirm without any doubt whatsoever, belt and braces, 110%, that the 1841 family in Lillingstone Dayrell is the right one, and that's to purchase the birth certificate of one of their children which will confirm the mother's maiden name. Suggest Job would be the best option, as there's a birth registration in Buckingham registration district, which includes Lillingstone Dayrell, in the Dec quarter 1850, vol 6 page 374. He appears on the IGI as being baptised in Potterspury on 22 Dec 1850, the son of Thomas and Elizabeth, but this entry is very unlikely to give you the mother's maiden name.
If there is a god, please make sure it's Read!
Northamptonshire is not the easiest county to research online as the LDS was refused permission to film many parish registers, which is why coverage on the IGI is so sparse.
I wouldn't get too hung up on census implied birth years, which often (usually!) vary by a least a year or so, one census against another.