Hello,
I have checked the Great Chart register and no William Bourn(e) was christened there during the period 1750 through 1785. Neither is he at Little Chart, Westwell, Ashford, or Pluckley. There is one christening of a William Bourne at Bethersden as follows:
BOURN William baptised 28 Mar 1773 to parents William/Eleanor at Bethersden , Source: Bishop's Transcripts.
However, if this is his marriage you may want to turn your attention to the Kennington parish register:
BOURN, William married DOWN, Charlotte on 10 Apr 1802 he bachelor of Kennington, she spinster of this parish after banns. Marriage at Ashford. Source: Bishop's Transcripts.
I hope this helps if you have not already found this reference.
Kind regards,
Susan