If you happen to have a golden, mellow, more complex tequila in your drinks cupboard, this is the time to use it.
Frost the glasses with the lime juice and some salt. Measure all ingredients, except for the 2 large spoons of crushed ice, into the blender jar. Whizz until slushy. Taste for sweetness before serving. Pour into small tumblers or margarita glasses, and serve with a spoonful of ice in each glass.