YESS! Finally, some success (of sorts).
Used the other driver and copied exactly the setup in that video and using the code they linked to.
It worked first time.
The stepper is not behaving the same as theirs, and for a lot of the time the motor shaft is simply vibrating at different intensities rather than spinning (it did briefly turn a little before doing this), but I'm sure that must be down to the motor spec being different to the one in the video, or potentially the driver output as I've only roughly set it on this second driver.
Anyway, I then substituted the driver board for the original one and it did absolutely nothing.
So it's possible it was faulty, or it's possible (likely I suspect) that I killed it with earlier attempts. Either way, I'm glad these things are so cheap! Might order a couple more in case of further errors of judgement and since I ultimately want more than one stepper in use anyway. I still have an easy driver en route too.
I think next, I will do the following:
1) Check then set the vref correctly and try again to see if the motor turns better.
2) Possibly mess with the code a little to see what the motor does
3) Run your code again to see how it behaves now.
4) Possibly revert add the potentiometer back and revert to the original code, although I'm slightly hesitant to do this in case this is what could have killed the original driver?
Can you see any issue with the original code and potentiometer setup, other than the lack of capacitor and the sleep/reset not being jumpered together?
Feel a lot happier anyway now I've got some snifter of progress at least. And I appreciate your input, thanks.