Mix custard powder, sugar, and a little milk in a bowl to form a smooth paste.
Heat the remaining 2.4l of milk in a saucepan until nearly boiling and stir in the paste.
Cook on low heat while stirring continuously until the custard thickens then let cool.
Prepare the jelly following packet instructions but do not refrigerate yet.
Slice the Swiss rolls and arrange them in the bottom of two large deep trays.
Scatter 1.5kg of strawberries over the sponge, pour over the jelly, and chill for 90 minutes.
Pour the cooled custard over the set jelly layer.
Cover and refrigerate for 30 minutes until the custard is lightly set.
Toast 50g of flaked almonds in a dry pan for 2-3 minutes until golden-brown.
Whip 1.5l of cream to soft peaks, spread over the custard, and top with toasted almonds.