Hi Iffy650
If I understand you correctly, this is very simple.
If the shaft rotates but does not turn the cog, it must be the bendix that has an internal fault.
The cog must turn with the shaft. It may slide along to the end first, but once it reaches the end it must turn.
On your starter, if you turn the cog by hand - does the shaft turn?
If not, then you need a new bendix.
I hope that helps