I did a little searching on the internet so for those of us that don't have castor sugar and double cream here are a couple of ways to make our own. (Personnally, I think I will try just using heavy whipping cream)
First castor sugar:
Castor Sugar Substitute
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By Erin Huffstetler, About.com Guide
Castor sugar may not be easy to track down, but it is easy to make. In fact, it's really nothing more than granulated sugar that has been ground to a super-fine consistency. Here's how to make your own castor sugar in seconds:
Ingredients:
Granulated sugar
Preparation:
1. Place granulated sugar in a food processor or blender.
2. Pulse until it reaches a super-fine, but not powdery consistency.
3. Allow the sugar to settle for a few minutes. Then, use in place of the castor sugar called for in your recipe.
Tips:
1. Sugar may etch your blender pitcher, particularly if it's made of plastic. If this is a concern, use a coffee grinder or spice grinder instead.
2. You can also make your own powdered sugar at home.
Double Cream:
from www.ochef.com
Double cream weighs in at 48% butterfat, or at least 6% higher than the highest-fat cream available in the United States — heavy cream or heavy whipping cream, which is at least 36% to 40% fat (the wonderful New England dairy from which we buy our cream produces a 42%-fat cream, although it doesn't brag about it). British recipes call for double cream because they can — it's available and adds lots of richness to your dish. Will your cake flop if you use heavy cream? Almost certainly not.
Can you get a higher-fat cream to use in the recipe? Yes, with a little effort on your part. The first step is to prepare a homemade version of crème frâiche. Combine 2 tablespoons of buttermilk or sour cream with 2 cups of heavy cream. Heat the mixture to body temperature in a pan, and then let the mixture sit in a non-reactive container (stainless-steel, glass, or ceramic) in a warm place for 24 hours or so, until it thickens.
Step 2 is to line a strainer with a triple thickness of cheesecloth or a wet napkin, fill it with the crème frâiche, and suspend that over a container in the refrigerator. The whey will drain out of the cream, leaving it with a higher fat content. By the time about a quarter of a cup of whey drains from your 2 cups of crème frâiche, you will have a fat content around 48% to 50%. It will taste a little more acidic than fresh cream, and you will have to judge for yourself whether that might negatively affect the flavor
I read on another site that adding a few drops of pure vanilla will counter the tart taste. On that site the woman used yogart and heavy cream as a starter and put it in her yogart maker for 24 hours. from www.jamieoliver.com/forum