Jonathan Creek: Christmas Specials (2008)
2 episodes, approx 90 minutes / 120 minutes, respectively.
What you do get are two feature-length specials: Black Canary (1998), which takes place sometime between Series II and Series III, and Satan's Chimney (2001), which takes place sometime between Series III and Series IV.
There's two new characters introduced; one in each of the two episodes. Firstly, there's Gideon Pryke (Rik Mayall) in Black Canary. Gideon is Jonathan's equal in some ways. He didn't become a regular but does make a subsequent appearance in the show, and if things had worked out differently he could've been a great recurring addition. He's woven into a story that involves someone from Jonathan's past reappearing and, of course, needing a head-scratchingly complex murder mystery solved.