An interesting subject I've been following for many years - it's also interesting to look at man made aircraft as potential UFO's. I revisited a book I bought in the 90s which had a description of a UFO looking like nothing the witness had ever seen flying through a valley in a remote part of the UK. She gave a very good description right down to the "alien" in the front. Reading the book again it was an almost exact description of the F117 Stealth bomber (right down to the position of the pilot) Maybe a clandestine testing mission?
Have seen a few strange things in the air - 2 stand out in particular - one in the australian outback near Pine Gap (a very bright light plummeting to the ground then reversing and shooting upwards at an angle) and one (bizarrely) at an airshow at RAF Valley which was witnessed by quite a few other people as well. The first I had no explanation for - the second was a large disk moving through clouds - my guess was a conventional airliner reflecting ice crystals (but i don't know if this would lead to it appearing as a disk)