Well I'll be buggered!
Firstly no it was not on the outside of the lense - I cleaned and double cleaned even though I could see no problem. Didin't help.
BUT...
I just got it out to have a look at the whole aperture/focus thing as suggested by Tasadam and the UFO is gone! It was there yesterday on top of Pillenger and now it's gone!
While I was using it on a track today I slipped and fell head first (pack and all) into the dirt. The camera got a fair whack as I tried to break my fall with my hand with the camera in it and turned on. It's currently got a fair bit of debris and dirt on it but no UFO in it's pics!

I'm baffled - I was surprised it still worked at all after it's 'trip', but it works better than ever!
Thanks for all the help guys, if my mysterious visitors from another world return I'll try and eliminate them with a rocket

!
But for now if anyone else has this same problem I suggest taking hold of your camera and punching it into some small dead myrtle trees and wet dirt as hard as you can, then jump on it chest first with a full pack and rub it in the leaf matter good and proper - seams to work a treat!
