Beat the remaining 150g butter and caster sugar until pale and creamy. Gradually add the beaten eggs. Fold in the flour with a pinch of salt. Spoon the mixture on top of the apples and spread level. Cover with a pleated sheet of baking parchment and foil, tie securely with string. Put the bowl in a large saucepan with boiling water that comes halfway up the sides of the bowl. Cover and steam for 1½ hours, adding more water if needed. Rest for 2 minutes before turning out and serving with the remaining warm toffee sauce.