Put all ingredients in lidded saucepan, bring to boil and simmer for 15 minutes. Take off the heat and let cool for 15 minutes. Strain (the rhubarb will have disintegrated) and cool the liquid. Dilute liquid approximately 3:1 to taste with seltzer / sprite / ginger ale, serve with ice and a splash of gin or vodka. If you are feeling economical you can also use the strained rhubarb pulp and have it with shortbread and cream / yoghurt. It's not hugely exciting, but perfectly respectable - and unless you put the mint leaves in a bouquet garni bag you may want to fish them out of the pulp.