Line a 1.2 litre loaf tin with a double layer of cling film allowing it to overhang the edges. Arrange rows of sponge fingers, widthwise, across the base. Sprinkle evenly with 3 tbsp orange juice.
Put chocolate in a heatproof bowl with the butter. Microwave on Medium for 1½-2 mins, gently stirring halfway through. Whisk cream with 1 tbsp icing sugar until it just holds its shape. Fold in the melted chocolate until evenly mixed.
Select 8-10 even sized strawberries. Spoon half the chocolate mix into tin, then put selected strawberries down the centre of the tin, pressing down gently. Spoon over remaining chocolate cream and smooth the top. Fold overhanging cling film over marquise. Chill for at least 24 hrs, preferably 2 days in the fridge until set.
Weigh out 250g of the remaining strawberries, roughly chop them and put in the food processor with 2 tbsp icing sugar and 1 tbsp orange juice. Blitz until smooth, then press through a fine sieve to remove the seeds. Chill until needed.
To serve, unmould marquise and cut into thick slices. Serve with strawberry sauce and a few reserved strawberries.