To make the filling, finely chop 300g of the largest strawberries and sprinkle over the caster sugar. Leave for 1 hour for the strawberries to give up their juice. Drain off and keep the juice, squashing the strawberries lightly against the sieve. Stir the mascarpone, condensed milk and strawberry pulp together. Line a 24 × 11 × 7cm loaf tin with baking parchment. Lay the loaf tin on the sponge and cut a strip just wide enough to fit in the tin, the length should fit as the sponge will have shrunk, but trim it if not. Put this in the base of the tin and brush the top with the strawberry juice. Add half the filling. Cut another strip of sponge a little wider than the last and add this, brush on the remaining juice and the remaining filling. Add the last piece of sponge and freeze the cake for an hour.