1: Put 300 g caster sugar and 200 g water into a saucepan. Bring to the boil on high heat.
2: Take off the heat and grate the zest of 2 lemons and 2 limes directly into syrup while it is still really hot. Mix well.
3: Cover the saucepan with plastic film, then leave to cool.
4: When the syrup is cold, pour in the juice of the fruit you have zested.
5: Mix well and your citrus syrup is ready.
6: If you will not be using it right away, pour the syrup into a bottle or jar which seals well and keep in the fridge.
Remarks
For the best flavour, it is important to respect the temperatures: 1) Add the zest to the hot syrup (stage 2) and pour the juice into the cold syrup (stage 4). If you don't like the texture of zest, filter the syrup through a fine strainer before use or bottling.