HI,
Eventually I did find the correct way of doing this.
First in my code, you must have a number in 'FastLED_fade_counter' in my case I use 255 as this fades from maximum brightness, and then in a separate part of code not shown here I check if the counter has reached '0' so all LEDS are OFF.
Either of the above numbers can be changed to suit your project.
Hope this helps someone in the future !
void Scene_fade_down ()
{
for ( int i = 0; i < NUM_LEDS; i++)
{
leds[i] = CRGB::Magenta; // Can be any colour.
leds[i].maximizeBrightness(FastLED_fade_counter); // 'FastLED_fade_counter' How low we want to fade down to 0 = maximum.
}
FastLED.show();
FastLED_fade_counter -- ; // Decrement
}