Pour 40 ml of the reserved apricot juice into a medium saucepan. Add the lemon zest, mixed spice and caster sugar. Bring to a simmer, stirring until the sugar has dissolved. Add the pears and prunes and simmer for about 5 minutes, or until the pears are tender (the exact timing will depend on their ripeness). Remove the pan from the heat and gently stir in the apricots. Leave the fruit to cool, then chill, covered, until ready to serve. To serve, top each serving of compote with fromage frais and pumpkin seeds.