In an interview with Helen McCrory in which she discusses various roles she has done, she brieflymentions her costume for Harry Potter and the Half-Blood Prince, which is currently being filmed in the UK.
I play Narcissa Malfoy in the new Harry Potter film. If I told you what I was wearing, I would probably have to kill you; it's a bit like working for MI5. My costume is very couture, with beautifully sculpted silhouettes. The costumes are designed by my Parisian friend. Of course, style was battered into her at an early age.