Many thanks for your input to the 'mystery', Mike.
I HAVE confirmed 'the' Daniel Martin I am researching was married to Mary Wilkinson in St Margaret's in 1851 and his father is listed as John Martin (labourer). (Thanks to Linda for that piece of information.)
BUT:
Age in 1851 marriage registration:- 28 (birth year 1823)
Age in 1861 census (Leicester):- 38 (birth year 1823). Family details for this entry are otherwise correct, so I have the 'correct' Daniel.
Age in 1881 census:- 60 (birth year 1821). Again, family details for this entry are otherwise correct.
Given the "uncertainty" in birth details, it's POSSIBLE the baptism in 1818 fits, given the father's name (John). (Maybe not! If there's a small multitude of Daniel Martins, how many John Martins are there?!)
With the lack of a John Martin (labourer) as well as Daniel in the 1841 Sileby census information that you have provided, it's looking entirely possible that the family had relocated from Sileby to Leicester (possibly Savney Gate - that is the address given in 1851) by the 1841 census and that's where I may be able to pick up the thread.
Merry Christmas!