Online purchase only - go to this page and follow the instructions.
And they'll even give you your money back if you don't like it!
David Alden's production, which premiered a couple of years ago, is intelligent and innovative. I'm not familiar with the conductor, Eivind Gullberg Jensen, but the cast, which includes Amanda Roocroft and Tom Randle, sounds fairly reliable.
If you're not familiar with the Coliseum's seating, the balcony is only for people with teeny bottoms and short legs. Some of the dress circle seats have a reduced view of the top of the stage, so the side stalls are the best pick if you can spring for the £20.