Aha, thanks, I'll try again with the alternate hardware package and get back to you!
I'm not getting the rapid blinking at power up, which I understand usually signifiy that it has booted.
Instead the light is on for 1 second, off for 1 second, then on for 1 second, repeating endlessly.
Without any bootloader at all though, all it does is light on or never light (depending on the revision of the board), so it has uploaded something which is running some process. I suspect there's a conflict with the board package. The one you suggested hopefully fixes it.
pert:
but that is "Note for Arduino 1.0" and you're using Arduino IDE 1.8.5 so you don't need to worry about it.
Aha, I haven't been around since the early days so I wasn't sure what it meant by Arduino 1.0, did it mean the drivers? IDE? The revision of the actual hardware?
I assumed it might have meant you had to changed if you weren't using some early prototypes or something. Thanks for the clarification!