Seems my regular consumption of bloody marys, bullshots and bloody bulls (tequila, tomato juice, beef stock, spices) had slipped my mind. There's a lot of useful stuff on umami out there on the web (for those who aren't familiar, fermented soy, seaweed, parmesan, dashi, mushrooms, peas and tomatoes all have umami character), but I didn't know about grapefruits, which are one of my favourite cocktail ingredients. E.g. the mainbrace (gin, triple sec, grapefruit juice in equal measures, a lovely pale pink colour if you use ruby grapefruit). I'm still interested in the tamari martini though.