Summing up everyone's contributions:
The William Willis who married Ella Jenkinson in 1923 was born 22 Aug 1896 in Tiverton and gave his father as William Willis.
The William John Willis who was registered in Tiverton in 1896 died shortly after and so cannot be him.
In the 1901 census William junior is with Thomas Fouracre and his wife Jane, described as grandson. in 1911 he is with his father William (born Cullompton about 1875) who has a wife Fanny but they've only been married 4 years so she is not his mother. In 1821 he is in Garstang.
A William Sully was born in 1896 in Tiverton to an unmarried Mary Hannah Sully, who later (1900) married a William Willis who gave his father as Mark. Mary died soon after the wedding. There was a William Willis born in Cullompton in 1875, parents Mark and Jane (nee Hawkins). This seems to be the same Jane who was married to Thomas Fouracre by 1901. So it seems very likely that William Sully took his stepfather's name and became the William Willis we're looking for. Who his biological father was is a different matter.
Mark and Jane Willis also had children Annie, James and Tom all baptised at Cullompton, though somehow James was born in America. Jane's maiden name was Hawkins. She married Mark Willis in the Tiverton district in 1867. I cannot see them in 1871, presumably they were in America. By 1881 she was widowed, living with her children in Cullumpton. . Next door were a William and Mary Willis and in 1861 William and Mary had a son Mark, aged 20, with them, so they are probably her in-laws. She later married a James King but by 1891 was widowed again, Several Willis and King children are with her in that census but not William (1875). He appears to be working on the farm of a Charles Potter. She then married Thomas Fouracre. In the 1901 census her son James (born America) and grandson William are with Thomas and her, described as son-in-law and grandson. Son-in-law presumably means stepson (although James is a widower so perhaps he married a Fouracre daughter who died).
To check the William Sully theory I would suggest getting that birth certificate. If the birth date is 22 Aug 1896 that would confirm he became William Willis (who has that date on the 1939 register).
David
