Confectioner's sugar frosting: I've made it for ages, but this time it was gluey.

I don't think using cream in your frosting explains the results- because a lot of American buttercream recipes call for using it. Powdered sugar in the US often has a little cornstarch mixed in to help prevent clumping. How much depends on the maker, and that's one reason the frosting can take on a gritty or grainy texture. The other thing that can vary is how finely the sugar itself was ground. And since you are working with a product that can attract moisture, the amount of that in your powdered sugar, as well as the air in your kitchen- will vary. So each batch of American buttercream has to have liquid added in varied amounts. It's not as simple as sticking to a strict set of measurements or weights. Your frosting sounds like it's just a bit thick, and needs a little more moisture and beating time- to whip in some air and lighten things up. Try adding in a teaspoon of hot water and beating it another minute or two- and try again. Continue to add in water- or even cream, until you get the right consistency. Don't worry about exceeding the amount you "normally" use, you use what the ingredients need at the time you use them.