The Stoweminator is a cocktail made with Port, Vodka and Baileys.
This legendary drink is referred to when JP first meets Kingsley in the local pub gents toilets (S1 EP 1). JP offers Kinsley a swig of the drink in his hipflask which he describes the ingredients given above and names it the Stoweminator. The origins of the drink are unknown, but it is presumably concocted by "the boys" and colleagues from Stowe School that JP often refers to.
Series 1 - Episode 1