Possible Clues:

• Infallible
• Unfailing
• Sure to work
• Can't find anything wrong with this trick for parking, on top of everything else
• Guaranteed to work
• Infallible, as a scheme
• Nothing will stop sweet academic, that’s certain
• Take in coppers -- cover is solid
• Not admitting failure of Foreign Office, given mostly historic evidence
• Can't be misinterpreted, even by simpleton