Hello David My line is basically the same as yours, but I go in a different direction with Richard 1764, William's 1758 brother. William 1758 did marry Mary Penwarden born 1747, but their son, Richard born 1786, married Elizabeth Slowman, not Mary Penwarden.
I have traced the Yelland line back to John 1734, John 1709, John 1671, and John about 1640, all from South Tawton, Devon. This appears to be the originating place of my line of Yelland.
There is another line of Yelland, from the book "Feudalism to Freedom, 400 Years of Yelland Family History." It starts in South Brent, Devon, and goes to Cornwall and then to Australia. These two lines possibly link up somewhere in that generation with siblings or one generation earlier, as South Brent is only 20 miles from South Tawton.