Thanks for early feedback.
A loop command may work but I've been concerned that just using the delay function would mess up the duty cycle percentage which is crucial to motor speed setting. I've not seen the BlinkWithoutDelay function before so will look into that - thanks.
The ic555 solution, is my last-resort fallback option if I can't get the Arduino circuit to work. It consists of two ic555s in series, the first sets the frequency, the second the duty cycle. More details on this (and links to others) that people have posted are on MC-2100 Treadmill Motor speed control circuit
The motor controller board comes with a opto-coupler for isolation so should be protected (to some degree) from my breadboard testing.
If anyone has a "set very low PWM frequency" known working code that still allows for a wide duty cycle variation within it, please let me know.