Put the zest and orange juice into a bowl with the
sugar and leave to marinate for 1 hour.
Strain the
liquid into a clean bowl and add the crème fraiche
gradually, beating until thick. Fold in the yogurt.
Chill overnight.
Cut the trifle sponges in half
lengthways. Lay them in the bottom of a small serving dish, moisten
with the orange juice, and finish with a layer of
the
syllabub.