For sorbet, line a fine sieve with muslin and set over a deep bowl. Add yoghurt; fold up edges of muslin and tie a knot to secure. Refrigerate overnight to drain and yield 500gm labne. Meanwhile, bring sugar and 375ml (1½ cups) water to a simmer in a small saucepan and cook, stirring, until sugar dissolves (2-3 minutes). Remove from heat; refrigerate until chilled (3 hours or overnight). Combine sugar syrup and sorrel in a blender and blend until smooth. Place labne in a bowl; gradually whisk in sorrel syrup to combine. Churn in an ice-cream machine; freeze until required.
For lemon jellies, juice lemon halves, being careful to retain their shape; reserve 80ml (⅓ cup) juice. Place lemon halves in a saucepan, cover with cold water and bring to a simmer. Cook until softened (20 minutes). Carefully remove with a slotted spoon and cool slightly. Remove membrane, seeds and pith with a spoon, leaving skins intact (these will act as the shells for the jellies).
Place lemon halves in a 12-hole, 80ml (⅓ cup) muffin pan; set aside. Bring zest, sugar and 250ml (1 cup) water to a simmer in a small saucepan; stirring to dissolve sugar; remove from heat. Squeeze excess water from gelatine, add to sugar syrup with reserved lemon juice and stir to combine. Cool to room temperature. Strain into a jug, then divide among lemon halves. Refrigerate overnight to set.
To serve, fill 8 shallow bowls with ice, add a lemon jelly, top with a quenelle of sorbet and scatter with extra zest.