Once we started delving into making drinks, it seemed only natural to come up with our own tonic recipe. Quinine, which is derived from the bark of the cinchona tree, gives tonic its characteristic bitter quality. Malic acid, another ingredient, is a form of citric acid with a not unpleasant sour flavour; it helps to preserve cordials and also gives them a clarity of colour. I ordered some cinchona bark and malic acid online and began to experiment. There are some very good tonics on the market, so I’m not suggesting it is necessary to make your own, but this one will give you a particularly special gin and tonic.