Top and tail five grapefruits so they'll stand on a board. Cut down the side of each grapefruit, following its natural lines, to remove the skin and white pith. Over a bowl to catch the juices, cut in between the membranes to separate the individual segments. Dry the segments on kitchen paper and squeeze any juice from the skin and membranes into a saucepan.
Squeeze enough juice from the last grapefruit to make the juice in the pan up to 300ml. Add the sugar and chilli, bring to a boil, lower the heat and simmer until the sauce thickens and you have about five tablespoons-worth of juice left – this could take up to 20 minutes. Set aside to cool down, then whisk in the oil, lemon juice, sumac and a quarter-teaspoon of salt.
In a large bowl, put the grapefruit segments, onion, chicory, watercress and basil. Pour over three-quarters of the dressing and toss gently. (If it seems dry, add all the dressing; otherwise, save it in the fridge for another leafy salad.) Serve at once.