Interesting, I had a browse through the custom functions and there is this one in the 80d
C.Fn II -7
If focus cannot be achieved with autofocus, you can have the camera
keep searching for the correct focus or have it stop searching.
0: Continue focus search
1: Stop focus search
If autofocus starts and the focus is far off or if focus cannot be
achieved, the lens drive stops. This prevents the lens from
becoming grossly out of focus due to focus searching.
Super telephoto lenses, etc., with a wide focusing drive range can
become grossly out of focus during focus search, taking more time to
achieve focus next time. Setting [1: Stop focus search] is
recommended.
Even if [0: Continue focus search] is set, focus search may not be
performed when a wide-angle lens is used
I changed it to 0 as 1 was set and it seemed ok, but only tried it a couple of times as light was failing, I have no idea what my 7d was set too. I shall continue to check further. It's got to work, I'm doing cosford airshow on sunday,