Custard powder tends to be made to thicken up from mixing with milk so might not mix very well with a thick cream and could end up too thick ?
You could try mixing the powder with a little milk at first to create a paste (as the packet will tell you to do) add a little more milk to slacken it off, then do the rest with cream ?
Failing that you could make your own custard the cheats way.. (well I think its cheating because its my way and doesn't seem as hard work as all that water bath way)
Directly into a cold sauce pan, add 3 egg yolks mix in a tbsp of cornflour, mix in about 2-4tbsp of sugar (depending on how sweet your tooth), and a good tbsp of vanilla extract (or pod etc) turn the heat on low and gently mix in about 1 pint of milk (or in your case cream).
Steve
Yes you can make custard in cream, but if you are doing that, why not make your own, using corn flour (not corn meal)?
Try this one, Delia's recipes never fail; http://www.deliaonline.com/how-to-cook/sauces-and-dressings/how-to-make-custard.html You don't have to have a vanilla pod, you can use concentrate or even essence, though that last one is not as wonderful.
AskCy's method is the kind of things I do, short cuts. I make my cheese sauce in a very similar way, everything together!