I've added a contemporary article about the game, from Computer & Video Games. Not sure of the exact date, but presumably from 1984.Nice picture of the late great Terry Jones.
I've added a contemporary review by Keith Campbell, from Computer & Video Games. I don't know the exact publication date, as the page had been torn from the original magazine, but I'm guessing late 1982 or early 1983.
I've added a rare Acornsoft product leaflet which shows this game under its original "Defender" guise. As such, it must date to no later than 1982. The other games listed are Snapper, Monsters and Philosopher's Quest, Acornsoft's earliest games.
Amongst the same papers, I've found an early Beebugsoft "Software Library" product leaflet, which lists this game and several others, including Magic Eel.