Blackcatter, have fun, send us the video. All whistle players should have that joke in their collection.
Richard H, I've had whistles that refuse to come apart. Sometimes, as a last resort, you have to revert to brute force and ignorance. I once made a small tool with two pieces of wood with a hole (slightly smaller than the dimeter of the pipe) drilled so that each piece had a semiciscular groove that would clamp the metal part of the whistle in a vise without squashing the pipe. I used a very small pipe wrench on the plastic bit. The secret is not to try to twist it off but to turn the pipe wrench tight till it is almost to the point of turning then give the handle end of the pipe wrench a sharp tap with a hammer to break the seal. This tends to crack the plastic about 50% of the time(I did say it was a last resort), so if you really need to have a tuneable whistle you should buy at least two in the same key in the first place. The method above gets the metal pipe off in one piece, you then have to try to cut, bend, bash and saw the pipe to get the plastic bit off in one piece from the second whistle.
S&R's method with the ring spanner (wrench) will also work using the wooden vise jig to hold the pipe steady.
I really wasn't joking about the grinder, either. The plastic part has a stop inside it, designed to set the whistle at that key. Getting the plastic bit off only allows you to "flatten" the instrument. By grinding a little bit off the end of the metal pipe (the end that goes into the plastic part), you can then push the pipe farther in than it originally went to also "sharpen" the whistle.
Not every unplayable whistle is completely useless. I found an old C whistle was exactly the right diameter to release a "quick-fit" connector on the hose of my car's fuel pump!