Measure out 375ml of juice and weigh 75g of sugar (always use a 5:1 juice to sugar ratio). Tip the juice and sugar back into the pan and heat gently, stirring, until the sugar has dissolved. Stir in the lemon juice, then cool. Will keep in the fridge for two weeks. To serve, dilute with chilled sparkling or tonic water, or top up with sparkling wine.