 Why does Simon look for aspirin as soon as he gets to his office?
He sat down at his desk, passed his tongue over his teeth in distaste, groaned, fumbled in a drawer for the aspirin bottle. "Providence," Simon told her whilst fiddling with the aspirin bottle, "will provide." The client fussed himself with Betty's assistance into the seat, bug-eyed Simon, said finally, "You know my name, that's pretty good. It's my contention that within a century or so man will have solved the problems of immortality and eternal youth, and it's also my suspicion that he will eventually be able to travel in time. It takes roughly seven and a half hours from Gander to Shannon and I spent that time dreaming up material I could put into my reports to Mr. Oyster.
 He has a hangover from attending a festival.
 He is experiencing caffeine withdrawal and did not have time to stop for coffee.  We never learn the cause of the headache, we only know that it is severe.  He has a hangover from attending a festival.  He was out drinking with some friends the night before, and has a hangover.