Yeah, I had got that far already! The particles do all spawn with a different frame, but theres no animation on them...
I have either got them animated and all flapping at the same time, or as above, each particle different but not animated.
I realise the obvious workaround is 2 or more Particle2D nodes all starting on a different frame, but I can see how this would be useful later in my project, and potentially reduce the number of Particle nodes on screen at the same time!
Im suspecting this is a wee bit beyond Godots particles control just now, but if it IS possible, great....