I am making an assumption that John Slack, age 40 in the 1881 census, living at 20 The Lawn, with his wife Charlotte, 32, is the son of Gilbert & Ann Slack. His age seems to be a year or so off, but that may not be unusual. In 1861, the Gilbert Slack family was living at 26 The Lawn, so perhaps John stayed in that area when he married.
In 1891 they are at 8 Club Row, Mansfield. Their daughter, Elizabeth, is age 25. Nearby is John's widowed sister-in-law, Ann, and her three children who are still at home.
In 1901 this couple are ages 61 & 51, living at 90 Nottingham Rd., Mansfield.