I have specified the Pins in the script closely following the Scott Marley
code:
(Arduino powered LED Mask with FastLED [Part 1] - How to write code for an irregular matrix - YouTube)
#define LED_PIN 6
#define MODE_PIN 3
The code works on the UNO perfectly as does the button mode change. The Mega works the full LED matrix as a "long string" with no problem.. I must be missing something basic... like sleep ATM .. I try specifying other Pins tomorrow though