I see that he had a child aged 10 in 1871, born in Baumber. This is unfortunate as the 1861 census for Baumber doesn't seem to exist any more. Part of that Census book is missing.
Further complications-
1) MARSHALL is a very common name in Lincs - over 500 in 1851 - too many to look at them all.
2) There are at least 3 WALCOTs in Lincs.
I'll look at the Walcots in 1841 in the hope of finding him with his family (and to establish which Walcot!). If I can find him there, I will try the same place in 1851.
UPDATE: the Walcot near Billinghay has three isolated Marshalls, all a bit older than Chas, all single.
UPDATE 2: the Walcot near Alkborough has no Marshalls. Alkborough itself has John + Jane and a Jane, all aged 60.
UPDATE 3: Walcot (south of Sleaford) has no Marshalls.
It shouldn't be too long till Ancestry put 1851 online; it should be easy for someone to find him then.