The cast was not available for a long enough time in 2006 for an entire series to be filmed, so a 2-hour special was made instead.
Jonathon is played by actor Chris O'Dowd. He and Katherine Parkinson (Jen) would star together not a year later in The IT Crowd (2006)
When Louisa's father refers to Joan (talking to Louisa) as a "meddling cow" it's not a knock on her weight, as it would had this been set in American. In England, Cow is a term for a lowly woman. In fact, there's a 1960's movie titled Poor Cow starring Carol White, who is a gorgeous blonde actress who resembles a model.
The only vague time it's mentioned that PC Mylow wasn't around was when the baker said there wasn't any law around when Doc Martin wants to shut down the contest.
Black climbing rope, such as that used to lower Doc Martin down the cliff face, is never used due to the absorption of UV which would weaken the rope. The rope used was a repurposed batch from a quarter ton purchased by a Middle East county for training troops in night time helicopter warfare.