Elderflower Cordial

In a scrupulously clean pot, bring the water to a rapid boil. Slice the lemon thinly, and place in a large bowl (Pyrex is best for this, or glazed), along with the elderflower, sugar and tartaric acid. Pour over the water as soon as it has boiled, and stir until the sugar has dissolved. Leave the bowl to cool down, then cover it and place somewhere shady and cool for four days. Using a well-cleaned spoon, taste to ascertain whether it requires further steeping. At the most, another day should be quite sufficient. Strain the cordial through a steel or plastic sieve that is generously lined with muslin, and pour into very clean bottles. Store in a quiet, dark place. Drinking can take place immediately, accompanied by the likes of vodka, vermouth and gentle lemonade, though the cordial can sit for quite some time, if given a chance. Makes 1 litre