I have just looked at the original parish register for Westbury on Severn for 1 July 1759 and there is the following entry:

John son of a stranger born of Thos Marshal.

This could be the baptism you are looking for with the stranger being Sarah Ball. If so then you also know the father`s name.

The other possibility is that the Westbury is Westbury on Trym which is near Bristol, I note that the IGI doesn`t contain the baptisms for this parish.