It is believed that William married in September 1878. Unfortunately, FreeBMD doesn't have his wife's name.

It is understood from direct descendants of William that although he was baptised as William Knibbs, he later reverted back to his mother's surname, BUTLER. I am told that he continued to be known as William Butler for the rest of his life.
When he reached pensionable age, he was unable to receive a state pension under the name of William Butler as his birth certificate clearly showed him as William Knibbs.
He chose not to receive the state pension rather than call himself William Knibbs, so I can only assume that he had very bad memories of his time as a "Knibbs". He clearly didn't want to be one.

William appears in the 1901 census under than name of William BUTLER, aged 53, living in the parish of Cheslyn Hay, Staffordshire with his wife Ami and 4 children.
William Butler 23 Staffs Gailey Staffordshire Cheslyn Hay Coal Miner Hewer
Ami Butler 50 Staffs Chorley Staffordshire Cheslyn Hay
William Butler 53 Oxford Staffordshire Cheslyn Hay Coal
Ellen Butler 18 Staffs Gailey Staffordshire Cheslyn Hay
Frances Butler 16 Staffs Cheslyn Hay Staffordshire Cheslyn Hay
Joseph Butler 11 Staffs Cheslyn Hay Staffordshire Cheslyn Hay
Grace Butler 3 Staffs Cheslyn Hay Staffordshire Cheslyn Hay

Kevin Green tells me:
"My g.g grandfather William BUTLER would not talk about his past. My next fix on his location after 1847 is his marriage in 1872 in Wedges Mills near Cannock, Staffs. One family story I have heard was that he would not claim his pension after retiring, I assume because he needed a full birth certificate. He managed to live to the age of 88 yrs and outlived his eldest son by 10 days in 1935"