When trying to observe 'unusual' airborne activity it is VERY important to stop and remain in the same place, this applies when driving or walking.
Find a static reference point, a TV ariel or a tree, and watch how the object moves in relation to it. It is only too easy to 'see' a planet move when you are moving too.
There are a lot of strange things out there, chinese lanterns, stealth aircraft and indeed UFO's ie. just unidentified.
Providing you are static and it makes strange manouvers then it probably is not a conventional aircraft.
I have never seen anything strange as PIC but many professional pilots have. If you have a camcorder make sure that the auto focus is turned off, otherwise the object will appear to be growing and shrinking, which sadly many amateur UFO vids show.
This is a very interesting topic and I'd love to know what the 'Gurnsey' pilots saw.