No publication date given, but probably June 1979 (Beholder was a monthly zine and an article in Demonsblood 1 (Jan 79) notes that the first issue of The Beholder was expected in April of that year. Guy and Mike were one of the few UK fanzine publishers to stick to a monthly publication schedule.
From the contents list:
Page 2...Editorial & contents
Page 3...Magic System
Page 10..Monster summoning (new D&D monsters)
Page 14..The Villar of Menopolis (complete dungeon)
Page 24..View Point: Languages
Page 27..Contact and Feedback