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.
I went to the company in Sheffield that RAB recommend. From recollection it cost about £30, but for a £200 sac that seemed worth the piece of mind in a job well done. Rab are keen to warn you in no uncertain terms about the pitfalls of washing it at home.
I always use one of those slinky silk liners now, in order to keep the bag fresher for longer.
In reply to deanstonmassif:
There is no reason not to do it at home. The only thing is if you need to move the bag while wet (ie to a separate drier) Then just handle very carefully to avoid ripping the baffles.
Handling a 4 season bag is different due to the bulk it wont fit in most washing machines and is very heavy when wet. Hence best to send it to professional cleaning.