Q: Could you tell me what castor sugar is? I have no idea what or where it is.

Caster (or castor) sugar is the British equivalent of what is called Superfine or Berry sugar in the U.S. and Canada.  It has a smaller grain than regular sugar and is usually called for in recipes where the sugar needs to dissolve easily.

If you can't find superfine, whiz the same amount of  regular sugar in your food processor until it is broken up, but not powdered like icing sugar.  Don't use icing sugar in place of caster as it has cornstarch added to keep it from clumping, which may affect the recipe.