Thanks all for the nice comments :-)
Geez, Guppy what a great shot! I was using a painted profile as a reference for that presentation Spit, and that photo is a gem.
Stoney, you've nailed exactly what I was going for, namely uniformity. Weathering is very subjective and I'm not very good at certain things like oil streaks, which Merlins produce like toddler's chin dribble. I try to do enough to make it believable yet it's easy to go overboard and ruin the effect. Same goes for cordite staining, depending on the aircraft in question and the theatre/time period it was active you can have no stains at all or in the case of the BoB, a very weathered appearance as everybody was more concerned with operational maintenance vs. parade square spit 'n polish.
For the overall paint wear effect I use two 'noise' layers, one black and one white. Combining them and adjusting the opacity gives a a nice uneven faded look. You can erase or tone down certain parts of the layer if you want the appearance of smoother areas like where armourers have been sitting/kneeling on the wing for example. There's a lot of weathering layers though, this template has a total of about 120 layers at the moment!