I thought I was nearly there.
The motor seems to want to run. Initially it was just nodding so I checked the windings, which are fine. I increased the current limit to max on the A4988 but still not right.
When asking for sidereal rate tracking the motor spins about 5 times too quickly, but this is deceptive as it's not stepping, just vibrating round. There is no holding torque so the shaft can be turned by hand in either direction. A slew causes the motor to notch back and forth and then stop, but then the motor produces the sound I would expect as it ramps up. The sound remains until it slows at the end of the slew and then notches, and stops.
I'm not sure where to go from here.
Motor was removed from a working PICgoto system, and the A4988 was known to be working on a smaller stepper. The motor is a Longs Motor NEMA23 8 volt 1 amp unipolar, connected as a bipolar with the centre taps unconnected. The A4988 should be easily capable of driving this.
I've reloaded the firmware and configuration.
Any advice or suggestions greatly appreciated.
Phill