Ran out of Cointreau so used the next nearest thing I had: Italicus (!).
50ml Reposado; 50ml Blanco;
50ml Italicus; 25ml Agave syrup;
Juice of one small lime; small pinch of salt crystals; five thin rings cut from the end of a (relatively) mild red chilli; shake well with ice (as I wanted to dilute this a little). A surprisingly drinkable and slightly piquant margarita!