From sometime after 1977: THE PROFESSIONALS OFFICIAL POSTER MAGAZINE, part of the TV SUPER HITS series, based on the LWT production for ITV.
This is Number 4 is the series but I suspect the numbering belongs to TV SUPER HITS rather than specifically the fourth issue dedicated to this series. I'm sure I've seen issues devoted to other shows (the edition I spotted on display, for a price I wasn't willing to pay, featured THE DUKES OF HAZZARD) so I suspect this was a flexible umbrella title that allowed the publisher to target whatever show looked hot at that moment.
Network are currently releasing the show on Blu Ray disc... and the season box sets are well worth snapping up. Not only do the shop-on-film episodes look gorgeous in their fully restored glory (I doubt the prints have ever looked so fine... the restoration work is amazing) but each set also comes with an incredibly detailed book by Andrew Pixley devoted to the making of each season. Londoners will also delight in some gosh-hasn't-it-changed location spotting as well.