One of the reasons many have given for Desmond`s disappearing act in Live and Let Die is that the producers had wished to downplay the ever increasing reliance on gadgets in the films, and thus kept Llewelyn from appearing. Logically, that does not make very much sense, considering Bond uses an inflatable pellet gun and a buzzsaw/magnetic wrist watch.
Desmond answered the question in the July 1983 issue of Starlog Magazine: “Since I first appeared as Q, I`ve worked in all of the Bond films except Live and Let Die. When that film unit went to New Orleans on location, I was contractually involved in a weekly TV series and couldn`t leave England.”