In a large pan, gently melt the sugar in 250ml water. Meanwhile, using a vegetable peeler, pare away a few long strips of lemon zest. Squeeze the juice to give about 100ml.
Add the zest, juice, gooseberries and mint leaves to the pan, and bring to a simmer. Cook for 5-10 mins until the gooseberries are very soft, then mash with a potato masher and leave to cool. Strain through a sieve, pressing the flesh well to extract the juices, then chill thoroughly.
To serve, dilute 4 tbsp of the lemonade base with 100ml sparkling water (or to taste), adding extra ice, lemon and mint.