Trim and wash the rhubarb and cut it into 2.5 chunks. Place in a baking dish, sprinke with the golden caster sugar, and add the chopped fresh root ginger, then cook in the oven, for 30-40 minutes, unti tender.  After that tip it into a sieve set over a bowl to drain off the excess juice. Puree the rhubarb along with 2 tablespoons of the reserved juice.  When cold, fold half the puree into a bowl containing the Greek yoghurt, then divide it between 4 serving glasses and spoon the remaining puree on top. Finally, cut the stem ginger into matchstick lengths and use them to garnish the fool.  Cover and refrigerate until needed.