This morning I grabbed a scribing tool from the workshop and ran it down all the “blank” sections of the stripboard on the first board a few times to try to clear out any tiny drops of solder that might have been bridging tracks. I also found one place where my soldering might have unintentionally bridged two tracks though I couldn’t tell for certain. If there was a bridge then it would have pulled the RESET (pin 3) input to ground. As that input is active low I imagine it could well have caused the behaviour I was seeing. I removed the solder using some braid, cleaned between the tracks and remade the join just to be sure. I now have a Nano sitting in the socket and behaving as I would expect 🙂
Now as if it’s not enough having to deal with my eyesight not being what it was in my youth, I can’t remember where I put my stepper drivers either 😀