THAT letter was deemed a hoax
All the researchers discussed it.
The writer was just looking for publicity. He went from the Avebury Team to the Amesbury team in the same letter. There are only 2 airports in the area where these formations are going down, and all the pilots know each other as they are small airports. There's a few light aircraft, but mostly just helicopters and the home built ultra-lights.
I'm pretty sure I posted that when I found out.
I've just posted an open letter to the media to out these hoaxers once and for all and when it makes headlines, you'll be able to read it.