First of all, mix the jelly according to the packet and sprinkle two teaspoons of gelatine on top, whisking through. Pour into the mold of your choice or leave to set on a tray. (you will have a lot more than you need- so set the rest in other molds unless you are making lots of trifles)
Put the savioardi biscuits in the bottom of your dish, cover with the juice.
Add the sugar to the greek yoghurt and whisk through.