Ok, so I've seen a few recipes that require both baking powder and Bicarbonate of soda (baking soda).

I've only ever used baking powder, as a rising agent....will bicarb make much difference?

Anyone have experience of this...

Ta muchly

c
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When a recipe contains baking powder and bicarbonate of soda, the baking powder does most of the "rising".

The bicarbonate of soda is added to offset the acidic ingredients like sour cream, cocoa, etc in the recipe. It also acts as a tenderizer, making a "softer" crumb. It may also add to the rising power of the leavener, but this is not it's primary action in the recipe.
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Yes, I have experienced this also, just as auzzi has said, I find it where cocoa has been used in a recipe. (or buttermilk, or sour cream)

Baking is a science of chemical reactions. If it is written in the formula, (recipe) follow it.

Still in doubt?
Try the recipe with and then without the baking soda. You will probably find the product without the baking soda, flatter and not as tall as the product with.

interesting...so what should the ratio of both be? I have a recipe where I use sour cream, buttermilk and cocoa powder and was using 1 tsp powder to 1/2 tsp soda. wondering if I could get a better cupcake messing with ratios....