Place 150ml/5fl oz of the water, 55g/2oz of sugar and the juniper berries in a pan and bring to the boil. Then remove from the heat and leave to infuse for 30 minutes.
In another small pan, bring the remaining water and sugar to the boil, and remove from the heat in order to cool and form a syrup.
Place the segments in the sugar syrup, and chill in the fridge.
Strain the sugar, water and juniper berries through a fine sieve into the grapefruit juice, then add the gin.
Pour this into a wide and shallow metal container and place in the freezer for about 1 hour, or until the granita starts to freeze and form crystals. Scrape the sides and mix gently to combine more frozen parts with the less frozen.
<strong>To serve:</strong> place a few of the grapefruit segments in each glass. Scoop a spoonful or two of the granita into the glasses and top with a few more segments. Pour a tablespoon of the sugar syrup used to marinate the grapefruit over the fruit and granita and serve at once in pre-chilled glasses.
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James Martin is joined by top chef Tom Kitchin for the cooking show.
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