Ian is the father of Sophie and Jamie, and the husband of Penny. He first appears in the episode Sophie's Parents, where he is shown to have drinking and anger problems, and a dislike for Mark, even before he discovers that Mark doesn't properly love his daughter.
He is then shown in the episode Wedding when he attends the miserable wedding of Sophie and Mark. Later he is shown in the episode Spin War when he picks Sophie and her cousin Barney up from a gig that Jeremy's band is playing, yelling at Mark after he finds out that Barney has given oral sex to Super Hans.
Ian Chapman is known to be "a homophobe, but he's no badger baiter"
Ian also appears in the episode, The Party, when he arrives at Mark's party to discuss the legal matters of Sophie and Mark's unborn child. Ian makes an offer to Mark's Solicitor (and Sister) Sarah to have Mark come and work on his farm in a cushy but well paid job, as well as free accommodation in Sophie's Nan's cottage and £45,000 a year if Mark agrees to settle down with Sophie and raise their child with her. He seems confident he can persuade Sophie to agree to this but Mark dramatically refuses the life of a Country Farmer.
His last appearance is in the episode Nether Zone, when Mark misses his son's christening, and is greeted by an angry Ian at the door when he finally arrives. In this episode, it is discovered that the baby has been named Ian James after Ian, instead of James Ian, Mark's first choice for the name.