I think it must be a short somewhere between some of the tracks. This evening I’ve built a new board from the ground up, plugged in the same Nano and stepper driver and it’s running “blink” fine. A small part of the layout is slightly different from the first one as I realised I could do it a little more neatly, but the circuit is clearly the same. I guess I just need to get the big magnifying glass out so I can check things over better.
At least I have something I can continue working with now.