Hi,
There are a few John LIGHT marriages to a Jane in the area about the right time, the closest to Fisherton Delamere that I can find is about 6 miles away,
Parish of Dinton
21st August 1797
John LIGHT of this parish
Jane BAILEY of the same
By Banns
Both marked X for there names.
Witnesses: John BAKER, John JESSE
John JESSE seems to have been a witness on a lot of these marriages so might not be a clue.
I don't know how you can be sure these are the right ones. Does it show his parents names on his death cert., assuming it is in Australia?