Hi Mr. Happy--

We met (briefly) in Middlewich in 2005...I can't believe it was 3 years ago!

I've got C, D, and G whistles, and here's what I've found...

Anyway, if you can play in G and A on the D whistles, you can comfortably do G and F on a C, D and E on an A, etc.
In theory, you have mastered the fingerings to play in the 1, 4, and 5 keys of the whistle in hand. You also can probably play in the associated minor keys--Bm for D whistle, Am for C whistle, Em for D whistle.

I've found that I can also play in the minors for the 4 and 5 keys: for example, on a D whistle, I can play in G and A. I can play tunes in Bm. I can also manage some tunes in Em and F#m.

On your Bb, then, you should, with the skills you already have on the D whistle, be able to play in Bb (of course!) and it's associated minor (Gm); the 4 (Eb), and it's associated minor (C#??? not sure what that one is); and the 5 (F) and it's associated minor (D).

I hope that helps! Have fun!

V