I accidentally burnt incorrect fuse settings into an Arduino MEGA.
Not sure what I programmed but it was dead, many avrdude attempts with various option settings couldn't coax it back to life.
Then I came a cross a post by "krupski" here:
https://forum.arduino.cc/index.php?topic=238792.msg1724937#msg1724937
His method is to use the clock from the 16u2 serial interface chip and feed to the 2560 external clock in pin. This allows the 2560 fuses to re-programmed.
Instead of trying to pickup the 16u2 clock at the resistor as krupski describes I soldered directly to an xtal pin on the back of the board (pin nearest middle of board), brought the wire to the front and soldered it to the resonator resistor as per the krupski instructions.
I was then able to set fuses to the default state and the MEGA was back to normal.
Well done and thankyou krupski!