Andy,
I had another look through my records, because it seemed strange that William appeared to be living with Isaac and Nancy on the 1991 census as a Bancroft, and yet Isaac and Nancy were living with just one daughter, Martha, in 1881.
As William was born in 1869, I would have expected him to be living at Farnhill Top as well with them at that time.
I looked around the area on the 1881 census, and guess what!
There is a William Teal, age 12,living at Cragg Side, Sutton [which is the next village to Cowling] with James and Elizabeth Teal. He is described on the census as their son, but they are 73 and 71 years old, so this cannot be correct.
I suspect that they were probably his grandparents, and he was either orphaned or illegitimate, and that by 1891 James and Elizabeth were no longer around, so he went to live with Isaac and Nancy who adopted him either legally or unofficially.
I would suggest that your next step should be to look up the 1871 census and see if William was living with James and Elizabeth then as well, or maybe living with his parents or an unmarried mother called Nancy? or even someone else in the Sutton area.
I can send you the Issac family line, but if may well not be needed if William turns out to be from another line.
Let me know how you get on.
Regards
Jarlath