The particular stemaed pudding that raised the question is a orange marmalade stemed pudding recipe by Gordon Ransey. It is intended to be a "sponge" pudding, not the really dense type like a Christmas or Plum pudding. My experience with the Christmas/Plum puddings is that the use of suet really helps lighten up what might otherwise be as dense as a brick.
I've seen this type of jam pudding recipe in various places on the internet and it often calls for both self-rising flour and baking powder.
Since asking the question I tried the recipe and used regular pastry flour instead of self-rising, and 2 tsp baking powder per the recipe. The result was indeed sponge-y yet still moist and slightly dense. It seems to me that the self-rising flour in a sponge-type steamed pudding that also has baking powder is completely unnecessary.