Try using boiling water straight from the kettle... leave it to soak and then the glue should break down.

I have done this to all my generation whistles and it works a treat.

You just have to make sure you get all the glue off from inside the fipple and around the metal. So clean it off thoroughly or it will stick again.

Only thing though, be carefull that the water isn't so hot that it melts the plastic. (add a small dash of cool water)

Ella