The Jupiter Myth is the fourteenth book in Lindsey Davis' Falco series.       Marcus Didius Falco is about to get involved in a Londinium noir tale of gangsters, gladiators and love.     For Falco, a relaxed visit to Helena's relatives in Britain turns serious at the scene of a downstown murder. The renegade henchman of Rome's vital ally, King Togidubnus, has been stuffed head-first down a bar-room well -- leading to a tricky diplomatic situation which Falco must diffuse.     One murder leads to others, and with his vigiles pal Petronius, Falco leads the hunt for gangsters who are intent on taking over, while danger and death lurk throughout their pursuit.