You have all been very helpfull for which you have my sincere thanks. I now have two high quality photos taken from 7 oclock low of aircraft in flight during the war. Although B&W photos, the quality is good enough and one is USAAF bare metal; the other Coastal Command white, both of which clearly show the staining.
Ironically in a thread about engine staining, the best bit for me is the knock on effect on the tail assembly. The vertical fins get really quite dirty which for me is music to my ears.
I am wondering what caused the staining on the outer portion of the wing on the Coastal Command aircraft photo above, especially as I doubt that CCmd aircraft engaged in formation bombing ie staining from a lead aircraft.
Can anybody confirm the normal status of the engine cowling flaps (Apologies if terminology is wrong; I am a turbine man). ie Are they only open at a particular stage of flight or indeed starting, or are they individually adjustable as part of a tuning process.