Machine (new to me) Sherline 5400 with cnc

Hi. Despite lots of searches, YouTube, Google, forums etc I have to admit I am stumped.
Had the machine a few days (second hand) and had good results with Mach 3 demo and some example gcode.
Yesterday, the limit switch at the top of the z axis failed and the machine didn't stop on a "zero" command.
Steeper motor still turns but does not drive threaded rod. I guess something has stripped BUT.... Can't work out how to remove the stepper. I have been gentle with it so far but don't know if I should be using a"puller" to lift the motor with a bit more force but want to ask before I do.
Click image for larger version. 

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Picture attached which I hope posts OK.
Click image for larger version. 

Name:	uploadfromtaptalk1428780023074.jpg 
Views:	449 
Size:	292.0 KB 
ID:	15130
I am also intrigued by what the small brass assembly does, I hope it shows in the picture.
Hope I have given enough information.
Thanks