Make the custard as per the packets directions or... heat 2 1/2 cups of milk on a moderate heat. When almost boiling pour in the the custard powder mixed with the remaining 1/2 cup cold milk. Continue to stir until thickened and starting to bubble, reduce heat.
Add the sugar and the bananas. You can serve straight away or let it simmer for a while allowing the bananas to get mushy an combine more with the custard.