Line the bottom of a 1.2-litre/2pt cake tin with baking parchment.
Drain the apricots, reserving the juice. Put the apricots in a blender or food processor together with the sugar and 4 tablespoons of the apricot juice. Purée until smooth.
Measure 2 tablespoons of the apricot juice into a small bowl. Add the lemon juice, then sprinkle over 2 teaspoons of the gelatine. Leave for about 5 minutes until spongy.
Stir the gelatine into half of the purée, and pour into the prepared tin. Chill in the refrigerator for 1 1⁄2 hours or until firm.
Sprinkle the remaining gelatine over 4 tablespoons of the reserved apricot juice. Leave for about 5 minutes until spongy. Mix the remaining apricot purée with the custard, yogurt and gelatine. Pour on to the layer of set fruit purée, and chill for 3 hours.
To serve, dip the cake tin into hot water for a few seconds, and unmould the delice onto a serving plate. Peel off the lining paper. Decorate the top with the sliced apricot.