Help! Toilet handle not alwasy flushing properly/handle slip

[CatLuver8]

Hi there,
My fairly new loo handle is slipping again/not always returning to correct position.
I've had a look in the cistern and all seems fine apart from the metal handle which inserts into the plastic bit has a small screw sitting in one of the plastic holes - doesn't seem to be doing anything? I cannot see the logic of this as surely the screw should go right through the plastic into one of the holes in the metal handle?
Also, a previous plumber has put a thin piece of metal in another hole in the metal handle, to stop the plastic bit from slipping off the handle.
Help as don't want to call plumber out again.

 

[kevplumb]

the screw just tightens down on the square shaft

if the handle is not returning

either

its binding in the bush through the cistern wall

or

the link to the syphon is not set right