"I wondered about that, too. In mid-1970's anyone even remotely into photography already used a xenon flash."
Yep, my older brother was into photography back then too and he, as an amateur, had a xenon electronic flash with light sensors, ie it trigger at the required level. He was also on the Ark Royal while "Sailor" was being filmed. Never heard any stories about the filming, but he was still aboard when Rod Stewart turn up with enough copis of his single "Sailing" (in blue translucent plastic) to give one to each of the entire crew. Much of the crew had already pissed off on leave as soon as the gangways were down, so those left on board took two or three each just in case it might have a future collectors value.