Barbara: Yeah, that's what I meant--with the clarification that in the key of D, the C note (ti)is actually C sharp. But you can play a C natural on a D whistle, which is fa in the key of G, by alternate fingerings, the simplest of which is to hold down the middle and ring fingers of the left hand covering the second and third holes closest to the mouthpiece (sorry, I don't know how the holes are numbered). But I'm sure you know this already. The point is, you can easily play two different diatonic scales on any given tinwhistle.

Bill D: sounds like you are going for breadth not depth, as I also seem to be.