Hi Sharon,
You will have had 2 years further experience since your original post, and may well be further on by now.
If William was born in Heversham and you suspect his parents were married there, have you tried looking in the PRs back from 1808 to see if there are other siblings?
Another alternative would be to find Sarah's age from census or burial records and see if it ties in with any baptism record.
Good
HuntingChris