Here's an old family recipe I used to make the cordial we had at our Sandringham Gardening meeting on the weekend
25g tartaric acid
1 kg sugar
1 litre boiling water
Peel just the zest (coloured oart ) of 4 lemons with a potato peeler and place in a saucepan with the juice of all lemons and remaining ingredients and bring to boil cool the strain and pour into sterilised bottles and keep refrigerated and add to water or soda water at about 1 part cordial to 3 parts water.
I have made this with grapefruit and limes also but adjusted the amount of juice as this differs between fruits.