John Finnemore's blog mentions that the original split for "Ottery St. Mary" was supposed to be Douglas and Arthur in the van, and Carolyn and Martin off on their own adventure.

So, since we don't get to see them bounce off of each other that often without Douglas along, give me an adventure with just Carolyn and Martin!

(If you want to make it shippy as well, I'm all for rare-pair love, but gen works fine.)