A sprig of fern is just ridiculous! It's an alkali and will react with the acidic coffee oils, and will just create a salt. To enrich it you obviously want something more aromatic, like leaded petrol, or if you cant find that, dodo. Try stirring the coffee with a dodo feather, or if your local museum won't let you borrow one, a terapin swimming (anti-clockwise) in the coffee for 2 1/2 minutes adds just the right amount of piquancy (and terapin poo).
But actually answering the original question, I've found double cream tends to go a bit weird, and seperate into little noodles/worms of cream, unless there is a bit of sugar in it, but I don't have sugar in coffee any more. I just add milk, particularly if the coffee isn't so great. But better/good coffee I can drink without milk, although I do prefer coffee with milk even then.