Mixer tap handle - can't remove on/off handle (grub screw issue)

Every few years I need to replace the cartridge inside my old mixer tab. It's the usual type where you lift the handle to turn on the water then turn left/right to mix the water.

There's a hole to access the grub screw underneath the handle. This grub screw pushes against the square plastic top of the cartridge.

Today I went to undo the grub screw - it was stiff at first but suddenly released yet I still can't remove the handle. I can't see the grub screw either!

I'm guessing one of the following has happened:

a) I undid the grub screw too far and it's dropped inside the recessed area of the handle, but even so I still can't lift off the handle, it won't budge even a little

b) I've stripped the grub screw (or broken it)

As the handle still won't budge I think the grub screw is still in place - it's hard to see inside properly, even with a powerful light and a small magnifying glass, but I can't see anything that looks like a grub screw any more, which is strange.

Let's suppose that I have somehow removed the grub screw, how can I lift off the handle seeing as it's somehow jammed in place?

Should I just try and drill out the area where the grub screw is inserted? If I do that I'm not sure what to replace the grub screw with of course.


Finally, what diameter is the recess for the allen key on a typical mixer grub screw? I thought I normally used my 2mm hex key (allen key) - is 2mm the norm?

Thanks

 

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