I only have the 1851 census index:
see page 31
Henry DENNIS, aged 64 b Potton, Bedfordshire
Mary DENNIS, aged 58 - place of birth not listed
On page 7, there is a little girl called Eliza DENNIS aged 4 & b in Eaton Socon
Your John Samuel DENNIS was born the year before Civil Registration began and his parents were fairly elderly when he was born if they married in 1818.
If you hear nothing from this list, try the rootsweb mailing list for Bedfordshire. It may be worth e mailing Jim Bundy [his particular interest is Eaton Socon]:
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