CARAMELISED CLEMENTINE TART

4 clementines, washed and dried • 2 lemons, washed and dried • 100g caster sugar, plus 25g for sprinkling • 4 British Blacktail Medium Free Range Eggs • 170ml tub double cream • 100g unsalted butter, softened • 100g icing sugar • 250g plain flour, sifted, plus extra for dusting • Finely grated zest 1 clementine • 3 British Blacktail Medium Free Range Egg yolks