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Fill a large pan with 2 litres water. Add the sugar, lemons, vanilla pod and powder, and bring slowly to the boil. Boil rapidly until the mixture has reduced by half, then remove from the heat.
Add the elderflowers to the pan and stir gently. Leave to cool completely, then pass through a sieve lined with muslin into another clean pan. Whisk in the vanilla seeds and store the cordial in sterilised bottles with a strip of peel and the vanilla pod added (see tips below). To make a refreshing drink, use 1 part cordial to 5 parts iced water.