Jamaican Summer beer cocktail

This is the most refreshing cocktail I've ever had, perfect for a Summer evening, or even afternoon. It is only beer after all (with the tiniest bit of rum). I adapted the recipe from the one I tried at The Rum Kitchen a few weeks ago, which, incidently, is a really fun place for a night out in West London

Ingredients

1 part ginger beer to 2 parts beer (Red Stripe is obviously ideal)

1 teaspoon (or more) Spiced Rum

Few drops of Angostura Bitters

Lime juice (about half a lime per big glass)