100g citric acid. From the chemist. You will get funny looks. Explain what it is for. It may not be in stock as it is commonly bought to bulk up drugs! Cost c. €2
2 litres of water
Collect flower heads when dry and not covered in insects usually in May. Elder is a common native plant but abundant, no environmental impact from collecting flowers. Put everything in a pot, bucket,saucepan. Soak for two days, stirring occasionally to ensure sugar dissolves. Then remove lemons and flowers. Sieve through muslin to remove small bits and insects. Bottle in clean sterlised bottles. Use corks in case of accidents if you intend keeping it for months. Otherwise screw tops. It also freezes well (in those ice cube bags). Dilute to taste.