I spoke to Richard at TTS Performance, he says he has seen over half a dozen of these button heads pulled off the lifter piece. It's a common fault.
He said they once fitted a new clutch and fitted the actuator arm in the position where you think it should go but it was too far round and pulled the button head off the lifter piece when they operated the cable.
Apparently the newer models have a mark to show the exactly position to fit the actuator arm onto the lifter shaft, the older models (like mine)do not.