Les!!!
Who else has TAPPIN and where?

As for WILLIAMS, remember mine come earlier ... WILLIAMS b. about 1768 Towersey, Oxon, m. Sarah TAPPIN b. 1769 Bledlow, Bucks, in 1792, Bledlow.
They had the following children all in Bledlow, if I recall correctly, so still think there is a chance your later ones might come from mine:
John, 1793
Thomas, 1795 (Harris' direct)
Elizabeth, 1796
James, 1798
Mary, 1803 (my great-great-grandmother)
Ann, 1806
Sarah, 1808
Christopher, 1812
Also Sarah's (1769) mother was from Princes Risborough....
I'm pretty lost in this hunt, so sorry if all the above has already been shown to amount to nothing.