Looks like I've been phased out. Today GW have released a new line of paints and Devlan Mud has been taken off the list along with other favorites like Boltgun Metal. more details about them can be found here. I'm going to have to pop down to my FLGS to make sure I've got enough of the old colours to keep going with.
I've already started making the shift to Vallejo Game colour. They go through the airbrush better but do require the use of a palette for painting, rather than just dipping your brush in the pot.