I have seen some recipes using cake flour. I can't get this in Australia, should I use self-raising flour or plain flour instead?
Cake flour is most commonly seen in American recipes. It is a finely milled, very low protein flour (usually 8-10% protien levels) which is used for cakes. It is also bleached, which affects the flour by causing the flour molecules to repel liquid, bind fats more efficiently and stabilize the gas bubbles produced by the raising agents. Theses factors can give a cake which rises more and has a fluffy, tender texture, particularly in cakes that have a high proportion of sugar in the recipe.
Most cake flour does not contain raising agents so is not self-raising, though one or two brands are "self-rising" which can cause confusion. Usually you can tell from the recipe - if the recipe states just "cake flour" and it includes raising agents such as baking powder and/or bicarbonate of soda (baking soda) then flour will be the "plain" (or "all-purpose") type.
The US edition of Nigella's book How To Be A Domestic Goddess does include "self-rising cake flour" in some recipes and in these particular recipes you can use regular UK or Australian self-raising flour.
Bleached flour is banned in Europe and Australia (we understand this is for health reasons). For most cake recipes using cake flour (non-self raising) you can use plain flour, or some people like to add cornflour (cornstarch) to help reduce the protein content. For 1 cup plain flour remove 2 tablespoons of flour and replace this with 2 tablespoons cornflour - in metric terms use 105g plain flour plus 20g cornflour per 125g flour in the recipe.
For plain flour you can usually tell the protein content from the nutritional information on the packaging and you could search out lower protien brands for cakes. The UK now has a product called "sponge flour" which is self-raising so should not normally be used as a substitute for American cake flour.