For the filling, soak the gelatine leaves in just enough cold water to cover, for 5-10 minutes until soft.
Make the strawberry puree if you haven’t yet, our into a wide frying pan and simmer over a medium heat for about 5 minutes until reduced by half and thickened.
Meanwhile, place the sugar for the sponge fingers in a medium saucepan with 200ml of water and bring to the boil on a high heat, stirring until dissolved.
Give it another minute before pouring into a wide baking dish. Leave the sugar syrup to cool slightly.
When the puree has reduced, stir in the filling sugar until dissolved then remove from the heat.
Squeeze excess water from the soft gelatine and stir into the puree until dissolved. Leave to cool.
Whip the cream in a large bowl until just stiff. Fold into the yogurt or fromage frais in another large bowl until blended.
Grease the tin and line the sides with parchment. Soak a few biscuits at a time in the sugar syrup for 30-60 seconds, tuning halfway, until softened but not soggy or breaking.
Carefully place upright against the tine edge, sugared side outwards. You will need about 30.
Soak half the remaining biscuits in the syrup and lay them flat in the tin, trimming to cover the base completely.
Gently fold the cooled puree into the cream mixture until well blended. Pour half into the tine and smooth with back of a spoon.
Soak the remaining biscuits in the syrup and arrange on top, trimming to fit.
Pour the remaining filling over, smoothing again.
Cover with cling film and chill for 3 hours or overnight until firm.
Carefully remove from the tin, place on a stand and peel away the paper.
Decorate with strawberries and mint sprigs on top.
Cut into wedges and serve with softly whipped cream.