In reply to ashaw:
I've washed mine many times in my own washing machine. The key to the clumping is in the tumble drying.
I use a cooler setting and every 10 mins take it out of the tumber and give the bag a shake. Better to shake the bag by starting at one end and just shake every 10 inches of the bag and work your way through the bag then put it back into the tumber.
Its hard to tell if the down has dried fully so your have to feel for clumps of damp down, just keep tumbling and shaking.
You can use tennis balls in the tumbler which help a lot to break up the clumps.
I finish off with a wamer setting to be sure i've dried out the down fully.
I've had the bag for years and its always been ok doing it this way.
Jason