Grease container. Chose container from small, heatproof mixing bowl of mix-master, coffee cans, or a 6-cup/1.5 litre pudding mould. Lid must fit tightly. If using mixing bowl, grease waxed paper and put over top, then add layer of tin foil and anchor it all with wide elastic band. As pudding will rise, fill container only 1/2 full. Cream butter and sugar. Add egg. Beat well. In separate bowl, mix flour, baking soda, baking powder and add alternately to creamed mixture with milk.
Gently fold in berries. Turn mixture into prepared container. Secure lid. Place on wire rack in bottom of large stock pot with tight fitting lid. Add enough boiling water to come halfway up side of container. Cover pot with lid and bring water to low simmer. Steam for 2 to 2 1/2 hours. Check water often and DO NOT ALLOW TO GO DRY. Remove pudding from pot and cool to room temperature before unmoulding. Note: Pudding can be made ahead to this point. It will keep in plastic wrap for 2 days in refrigerator, or 2 weeks in freezer.