Hi bathroom specialists -- can anyone help? See picture
This chrome diverter sends water to the overhead shower or the bath. The square knob has lost its thread so just pulls off and does not stay on to pull the internal spindle back to bath position. The changer has always been quite stiff. Could anyone identify the part and let me know who makes it and whether this could be repaired or replaced with an alternative part that will grip the spindle. Otherwise will I have to have someone break off the tiles, dig into the wall and replace with a new fitting?