1) Place the beetroot in a large pot of boiling salted water and cook uncovered until the beetroot are tender, 30 to 40 minutes. Remove the beetroot to a bowl with a slotted spoon and set aside to cool. Strain the cooking liquid through a fine sieve and also set aside to cool.
2) In a large bowl, whisk together 350ml of the beet cooking liquid, the chicken stock, sour cream, yogurt, sugar, lemon juice, vinegar, 1 tablespoon salt, and the pepper. Peel the cooled beetroot with a small paring knife or rub the skins off with your hands. Cut the beetroot into small to medium dice. Add the beetroot, cucumber, spring onions, and dill to the soup. Cover with cling film and chill for at least 4 hours or overnight. Season, to taste, and serve cold with a dollop of sour cream and an extra sprig of fresh dill.