(It was posted 3 years and 11 months ago, which would make it 2013).
According to this author, all of the episodes were recorded one after the other on the same day. Benedict Cumberbatch left after his final scenes as he had to get on a plane for somewhere.
It is really a fun read - the cast clearly had a good rapport.
I didn't realize there were quite so many flubs, but of course they'd all be edited out before going over the air (probably several months later). [This is a far cry from the Paul Temple mystery radio series of the 1950s and 60s that I love. They recorded one episode a week, and it went out live!]
From the anecdotes recounted I wasn't sure what episodes were being talked about, but I do assume it was the fourth, and what they probably thought was the last series of the program.
Here's that URL again.
You can buy the complete series here: