The reason Sigourney Weaver provided a cameo is she's a lifelong friend of Selina Cadell, Mrs. Tishell. Plus Weaver was a fan of the show.
One of the patients is named Alice Cooper. A common theme is to name characters after famous actors and singers as an inside joke. A former patient (in Season Four) was named Ted Nugent.
The patient who cancels just before the American tourist is a Mr.Bellend. Martin is furious that he's canceled again. Bellend is a play on words. Bell End is English slang for the male anatomy.
In Facta Non Verba, S7E7, the American Tourist who annoys Doc Martin (Martin Clunes) is ably played by Sigourney Weaver.
Bert has a clever new way of making whiskey involving wood chips. The writers really should have been a bit more specific. The first step in whiskey making is to ferment grain just as if you are making beer. Insanely stupid moonshiners add wood chips to the fermenting vat to give the brew a little extra kick; the chips ferment into methanol (wood alcohol) which makes the hooch more innebriating, if you don't mind blindness, brain damage and death. The aging of whiskey is always done in oak barrels and there is a slow albeit complex interplay of whiskey & wood & air. The use of thin oak chips instead of barrels can give something resembling old style aging in a very short time.