mixer tap/shower problem?

'victorian style' like a tall telephone- do not know maker sorry, combined mixer tap and shower unit, controlled by handle which is pulled up or across to switch from mixer tap to shower. the changing handle has come off the tap stem, and won't go back in. it is a screw-in lever, with a sort of metal finger which engages in a slot or hole in a metal circular section in the tap stem -it goes in to the hole in the stem of the mixer tap, but won't stay in - there is nothing to hold in, and the screw head just goes round and round ,whatever is supposed to hold it in place has gone missing, so every time you pull the handle, it comes off. any ideas whats missing, or will i have to replace whole unit?