I am still no closer to figuring out what the problem was but now the Ego is working fine.
Took the board out to check for damage or paint/water etc on it, loose wires.
And I found nothing. Looked near brand new.
Put board back in and used the battery my team mate had put in it and the Ego worked fine, turned on fine, shot fine.
I spent a few hours turning it on and off again randomly and no issue at all.