Trim the rhubarb stalks and cut into chunks, then put into a medium–large saucepan. Tip over the sugar, add the ginger and lemon rind and pour over the measured water.
Heat over a low heat and bring slowly up to simmering point, stirring occasionally. Cover and simmer for about 6–7 minutes until the fruit is almost soft but not broken up. Take off the heat and leave to infuse for about 30 minutes.
Tip the contents of the pan into a fine sieve over a bowl and leave to strain for an hour or so.
Taste the juice and add sugar syrup to sweeten if you like (but bear in mind that if you add a mixer it will sweeten the cordial slightly, too).
Transfer the poached rhubarb to a bowl with a couple of tablespoons of the juice to moisten.
Then pour the remaining juice through a funnel into a sterilized bottle or other airtight container, seal and refrigerate. It will keep for up to 2 weeks in the refrigerator.
To serve, pour into tumblers with or without ice and top up with soda water or (particularly good) ginger ale.