Where did the names Siegfried and Tristan Farnon come from?

Question

The British vet and writer Alfred Wight is best known for his series of semi-autobiographical stories written under the pen name James Herriot, recounting details of his experience as a country vet in North Yorkshire. In these books, he himself appears as James/Jim Herriot, rather than his real name of Alfred/Alf Wight, and his friends Donald and Brian Sinclair appear as Siegfried and Tristan Farnon.

The name James Herriot apparently comes from a Scottish goalkeeper whom he saw on television. What about the names Siegfried and Tristan Farnon? Where did these names come from and why?

Answer 1

Thanks to @rand-althor for finding the citation!

This article, an interview with the author includes a brief explanation of the choices:

Siegfried was taken from the heroic character of German legend who had inspired composer Richard Wagner. The strong personality traits of his associate had not been lost on Herriot.
"After I decided on Siegfried for Donald, Tristan became the obvious name for Brian," the writer said. Tristan was a young romantic. That seemed to fit perfectly.

It turned out that my “Norse” recollection wasn’t completely off-beam either: the German Siegfried character is, it seems, basically the same as the Norse Sigurd (Wikipedia). These legends (in their German version) inspired Wagner's series of four operas, Der Ring des Nibelungen (a.k.a. The Ring Cycle), of which Siegfried is the third.

The Tristan character on the other hand is found in another Wagner opera, Tristan und Isolde (English translation left as an exercise for the reader), which he composed as a diversion after completing the first two acts of Siegfried.