Forth of Crowland Lincs

[John Forth]

I have hit a wall with the marriage of a William Forth to a Mary ?. They lived in Crowland Lincs. I cannot find a marriage, there is a marriage of a William Firth to a Mary Buck but this does not ring true. The pronunciation of the name Forth is very strong and in the actual parish records it can clearly be seen as Firth so I have discounted it unless anyone knows different. Therefore I am trying to find a marriage. William and Mary had at least 5 children the eldest born in 1757 and the youngest, my g-g-g-grandfather, born in 1770. Williams father,also William was Married to a Bridget(t). William was baptised 30 Sept 1733. If anyone can help I would be most grateful.