While the "Japanese" Sage Derby mentioned in the Python skit is certainly made up, English sage derby is a traditional variety. Many of the scripts for the skit I've found list the cheese spelled "darby", but that's just because it's pronounced that way. The proper spelling is derby.

The cheese originated in the 17th century where sage was added due it's medicinal value. These days coloring is added to accentuate the green marbling.

The pictured specimen was labeled "Sommerdale Sage Derby", which I'm pretty sure doesn't exist. But it seemed like too much of a bother to get in the cheese guys face over it. I'm also concerned that this was a cheap knock off rather than a real sage derby. The texture and flavor of the cheese reminded me of budget brand mozzarella, and I really think basil was used rather than sage. We'll have to track down a few other versions and compare.

Personally I think it was a sad example of sage derby, but we'll just have to see. My research showed that what is considered "sage derby" is pretty broad. Most descriptions of the cheese say that it has bright green marbling. But the cheese maker who claims to have the oldest recipe (Fowlers of Earlswood) seems to just add sage to a normal derby cheese which doesn't lead to any marbling at all. Also, some versions have thick marbling like the one we tried, but others are almost completely green.

One other oddity is that most descriptions mention that it was traditionally part of the ploughman's lunch, but the cheese predates said lunch by about 100 years. And I haven't been able to find any recipe for a ploughman's lunch that mentions sage derby.

Sage Derby entry at CheeseDatabase.com.