One of these is likely to be his birth:

Surname First name(s) District Vol  Page
Births Mar 1846   (>99%)
Bennett William Wincanton 10 567   
Births Jun 1846   (>99%)
Bennett William Wincanton 10 572
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A possible marriage, the only William bennett married in the south of england in the two years before the birth of their first son:

Surname First name(s) District Vol  Page
Marriages Sep 1868   (>99%)
Bennett William Richmond S 2a 449   
Darnill Alice Richmond S 2a 449   
Jakeman William Richmond, S 2a 449   
Morris Ann Richmond S 2a 449