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What's the name of the parish you were searching which is 80 miles from London? Do registers for that parish and nearby parishes survive for the decade Henry was born? Are there any gaps? Are all entries legible? What evidence do you have that Henry was born in or around 1685?
What was Henry's occupation? Do you know names of any of his brothers or sisters?
I have a Gillett/Gillat connection in 1700's but in Lancashire. He was Catholic, born before local R.C. chaplains began keeping registers so there's no record of his baptism, nor the baptism of his wife, my 5xGGM.