Cheers for that.
That's plan B at the moment, depending on when I can find time to take it back. There doesn't seem to be anything physically wrong with it, just a lack of lubrication on the seal for some reason that grips the visor and makes it difficult to open as it tries to turn the seal inside out. When the seal's wet and can slide past the visor it's no problem whatsoever.
Just wondered if anyone else had experienced it, and if so how they resolved it, other than by replacement.