Thanks again for all of the feedback so far.
I must be missing something. I have investigated the Blink Without Delay function and some example circuits/code. They do produce a low frequency but they have the duty cycle locked at 50%. The only example I saw where the duty cycle could be changed actually then changed the frequency as well.
For this project I need a signal that has a fixed duration low frequency (unchanging so the motor controller recognises the signal) whilst also being able to change the duty cycle (over a 15% to 85% range to change the motor speed) within that fixed frequency. So far, I'm not seeing Blink Without Delay as the solution for this need. But I may have not spotted how to set up the code to do this.
If anyone can point me to Blink Without Delay example code that a) has a fixed low frequency and b) allows the duty cycle to be varied as above (say, with a potentiometer) without changing the frequency, that would be very helpful.