Interestingly, Ian Fleming was an author who name-dropped every lux brand that he could--which feeds well into the whole, tiresome what-fictional-character-wore-what-watch debate that has been going on for about three thousand years in WIS circles. Ian Fleming, in one of the Bond books, stated that Bond wore a Rolex. This was enough to launch a million fucking threads about what James Bond would wear with his Argyle socks.
John le Carre, who was an infinitely better writer than Fleming, never stated what watch George Smiley was wearing. This goes with the very fine writer Elmore Leonard, who, in something of a master class, told aspiring writers to never describe the physical look of his characters, unless it was important to the story, which it really never is.
So, what would George Smiley have worn in his wrist? We know, from the books, that he used a cigarette lighter, to light Karla's smoke in India when he tried to turn him--and that lighter was given to him by his adulterous wife, Ann. The lighter, which Karla nicks, returns at the end of Smiley's People, when Karla defects to the West and hands the lighter back to Smiley, who is there to take him in from the cold.
From this, we would expect that Smiley was wearing a watch given to him from Ann. And as the lighter was obviously not a standard Zippo, the watch could have been anything.