window handle not working on double glazed window?

hi
the handle on my window turns but will not open/close window. i have removed the handle and checked the spindle which is fine. i have discovered the fault is that the square thing the spindle goes into is rounded and worn so the spindle just turns in it and does not make the mechanism work.
how do i fix it , where do i get replacement parts and what are they called so i can locate them??