It was the stepper drivers.
John, those two ICs on the Arduino are not getting hot on either of my two broken Arduinos. I didn’t get around to checking the mosfets on the broken RAMPS boards yet. They are in a bag marked broken for now!
My mistake was that I had damaged the stepper drivers, and I fitted them on the new Arduino/RAMPS, which broke the new boards, just like the first ones.
On my third set of boards now, but with new stepper drivers, and all is fine.
Thanks for all the input, expensive mistake but I’ll put it down to experience.