Chop the rhubarb into roughly 1/2 in pieces and combine the rhubarb, sugar, zest and water in a heavy-bottomed sauce pan and bring to a boil. Lower the heat to a simmer and cook very gently for 20 minutes, stirring occasionally, until the fruit is soft.

Set a fine-meshed strainer (or a coarse strainer lined with cheesecloth)over a large bowl. Pour the rhubarb through the strainer until most of the liquid is in the bowl. Stir the solids around a little but don’t press down (so the syrup doesn’t get cloudy).

Carefully pour the syrup into a clean bottle, cover and refrigerate. It should keep for a few weeks.