Roughly bruise the stems of the elderflowers, checking for any insects. Put 2 litres (70 fl oz) water in a saucepan and add the elderflowers. Tightly cover the pan with plastic wrap and steep over low heat for 4-6 hours. Ensure the plastic wrap stays tight throughout this stage as you don’t want to lose any of the flavours. Pass the elderflower water through a fine strainer and allow to cool.