Mummerset

Mummerset was, according to one account but not others, the West Country county of England in which Stockbridge was located. Its local lord, the Earl of Mummerset, sat in Stockbridge Castle. (AUDIO: Castle of Fear)

Behind the scenes
Mummerset isn't real, but it is a "fiction that exists in real life". A deliberate corruption of "Somerset", Mummerset is real British shorthand for "anything that generally comes from the West Country". It's often used to indicate when an actor is speaking in an accent which is broadly from the West Country, rather than specifically from Somerset or Bristol or wherever. The fact that Big Finish uses the location heightens the comedy of the piece, because British listeners immediately know that the setting is a fake — just like George, Roland of Brittany, the legend of George and the green dragon, and even Stockbridge. It also helps to facilitate what Peter Davison calls the "Monty Python" nature of the script. (BFX: Castle of Fear)

Of course, the story never disclaims Mummerset as a fake — it's subtext for the knowledgeable listener — so in the DWU, there actually is a Mummerset.

Because Mummerset means "the West Country, generally", and Gloucester is often counted as being in the West Country, it's possible that Castle of Fear does not actually contradict Stars Fell on Stockbridge's original situation of Stockbridge in Gloucestershire. One could simply believe that it's being called "Mummerset" here because the story is set in the medieval past, when many British geographic names were different.