There's all kinds of steamed puddings as you know Brian. Which one are you talking about particularly? I would imagine that with no leavening it would be pretty dense. Ergo, if you use all purpose (plain) flour you would need to add an extra dose of baking powder.
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.