William and Mary lived in Tavistock, Devon.I am aware of this because I am researching another William Yelland who also lived there and keep trying to untangle the two.
I have the Devon FHS baptism and marriage books.It shows Elizabeth baptised 25 March 1818 with her twin Sidwell Yelland.
Their first child was baptised Mary Rockwell Yelland in 1814.The marriage books show a William Yelland married Mary Rockwell in Tavistock on 28 March 1778.
I had hoped that might help me find my William but it seems clear that this was the parents of William the mason.
There was also a marriage of Sidwell Yelland to John Walters in Tavistock on 1 June 1814.As Sidwell was the first name of the twin to your Elizabeth I presume Sidwell Yelland was your William's sister.
An Elizabeth Yelland also married a Henry Poolly on 18 September 1826.