Slice out the flesh of the watermelon and chop it into small chunks. Place it in a saucepan and pour in the water and sugar.
Cook over a medium heat for 15 minutes, until the watermelon has almost disintegrated. Use a potato masher or the back of a fork to mash any stubborn bits of watermelon that aren't breaking down.
Leave to simmer for 10-15 minutes or until the syrup is fairly thick and the liquid has reduced greatly.
Sieve it into a bowl and stir in the lemon juice. Pour into a jar, leave to cool to room temperature then store in the fridge.
for the fresh watermelon mojito
Muddle the ice, mint leaves and lime in the bottom of a glass.