Thanks Neale,
I have looked at Mary Byers but can't yet link her with William, Thomas or Ralph. Will keep her in mind.
As William & brothers were baptised at Hart in parish of Grindon I looked for marriages in the area and came across John Watson's marriage to Anne Young in 1762 in Hart along with baptism of their son John also 1762. I am just guessing this is my John.
Thanks again.