Found the following concerning the abrupt exit of John Hopkins:
On June 27, 2006, CarolynJ posted on [1] Digital Spy:
After Daniel Casey left, John Hopkins was in several new shows. Now, John has left and Jason Hughes has been on the last 2 shows.
Jason Hughes character, Ben Jones, stepped in when it was mentioned that John's character, Scott, was sick. Does anyone know if John is coming back? If not, why did he leave the show? We like Jason Huges, but are just curious as to the change in characters.
Later that evening Forum member Joe King replied:
Sorry to say, John Hopkins will not be returning to Midsomer. After playing Scott for 14 episodes, John decided to leave the show (at the end of Series 8) as he wanted to do other roles. Unfortunately, he only told the producer of his decision to leave a few weeks before filming started on Series 9, so there wasn't enough time to film a proper farewell episode for his character. The only on-screen explanation to Scott's disappearance is Barnaby's brief statement that he's off sick (in the first episode of Series 9). And as far as I am aware, that's the last we hear of him.