The actress was elegant yet on-trend in a one-sleeved black silk crépe dress with a pleated cream panel from Vionnet's pre-Fall 2010 collection.
The only thing the actress needed to finish the look was a slick of pink lippie and chic peep-toe heels.
Maggie was joined by the other cast members, including Nanny McPhee herself, Emma Thompson.
Having just returned from a holiday in Mexico with close friend Kate Winslet, Emma - who wrote the movie - looked sun-kissed and stunning in her chocolate Vivienne Westwood dress and Aruna Seth sandals.
Other cast members who attended included Rhys Ifans and comedian Bill Bailey, while Gail Porter, Jo Whiley and Trinny Woodall took their kids along for the fun-filled movie.
By Ruth Doherty