I've cracked it, although how I missed it when I posted earlier I don't know.
Living in Sawtry in 1851 was Thomas Levis 43
born Stanion, Northants. Also in the same houshold was John Levis brother widower 29 b Sawtry, and Robert Wright father in law ie step father.
The key fact that I missed (how could I have done?) was Thomas' birthplace of Stanion Northants (which he also gives in 1861)
We've been saying all along Dave, have you got the Levis childrens' details from the parish register, or from someone elses tree. It's now clear that Thomas wasn't born in Sawtry, as you stated in your original post, but in Stanion, Northants, and that's where I think you'll find the marriage of John Levis and Rachel. Shows how unreliable other peoples trees can be, and how you should always verify details with parish registers.
One thoroughly ashamed of himself David in the south of France
Reuben's brother John is another problem but will hit you with that one once this is fixed.
OK, hit us, but start a new thread