A little bit of (probably useless) information to add to Chris Gibson's comments.
The Indefatigable gas field is about fifty miles north east of Cromer in Norfolk, ie the southern North Sea.
Although it's relatively close to RAF Mildenhall etc, surely any super-secret machine would not operate so close to the major air routes across southern England?
I'll get my coat and hat and leave now....
...and stop calling you Shirley.