ColinB
Thanks for the comment.
I think that, as a matter of courtesy, I will always ask the owners permission even if it is evident that the photos are in the public domain. The only snag with this is, that when permission is with held, one cannot, in all conscience, ignore and go ahead anyway; as in my recent experience.
I reiterate, most people are happy to oblige if they are aviation enthusiasts. It is often the 'collectors' who are not.