I’ve just swapped the stepper motor and attached cog with the working printer.

My “dodgy” motor in the working printer prints perfect.
My “dodgy” printer with other motor behaves the same.

This puts me back to either the nozzle or PTFE tube, heating not working correctly, or a software issue where it doesn’t want to extrude whilst printing.

One thing I did notice tho is that when I ran the heater to clear the nozzle on the working printer, it allowed me to push out the filament at around 190c, whereas on my dodgy printer I have to leave it a minute at 220c before I can push the filament through. When I get home I’ll take off the nozzle and inspect the tube and look for anything blocking it. Strange how the “load filament” facility works though.