Update:
I removed the stepper motor and tried moving it unloaded - same problem
SIEG gave me some further tests to put the software in debug mode and disconnecting the motor from the driver - same problem in that the software hangs at the same point

Therefore it looks like a controller fault.

Regards a simple motion controller for rotating an ATC only which might be the best route:
Mach3 demo (since limited code would be required) with a Mach3 card like:
https://www.amazon.co.uk/Motion-Cont...NsaWNrPXRydWU=

or an Arduino controller with some development