The actor Dennis Christopher, who plays "Jack", is never credited as Dennis Christopher, the credits just say "and jack". This is because the identity of Jack is a secret (for the characters in the series, they don't know who he is), his identity is unmasked in the first two episodes of the 4th season, from this moment he is credited as "dennis christopher as jack" in the credits. (There are two episodes, where he is credited in his role as "Albert Newquay", in fact, "Albert Newquay" is "Jack", also revealed in the meantioned episodes).