The way it is done here is 2 tsp baking powder for every cup of plain flour to make self raising flour (cup is 250ml). Shake the baking powder well before opening so it mixes properly (it contains two different powders that start working when they get wet), and sieve the flour 2 or 3 times once you have added the baking powder so that it mixes through evenly.

Good luck with your roly poly!

Yes - to clarify, the quantities I gave there were not to reproduce standard SR flour, they were the ideal quantities for steamed/suet puddings, according to my baking powder pot. It gives different quantities for different types of recipe, so in theory it should give a better result than SR flour. This is not according to my experience, however, I'm just reading it off the pot.