In reply to Doghouse:
If it is a 2/3 season bag, I would suggest washing it yourself. I have done it a couple of times in the washing machine with Nikwax down wash, then tumble dried on low with tennis balls (it takes about 3 cycles to dry). The bag comes out clean, smelling fresh and with restored loft.
If you dont want to do it yourself, or its a big bag Rab recommends sending it to W E Franklins in Sheffield. I haven't used the service myself so can't comment, but Im pretty sure they would do a good job.
Hope that helps.