In 40 years at the pointy end, only saw one UFO. Lomapaseo. It was vertical. We were at 37000ft and it appeared to have no relative movement and was much higher. I was a young . lowly but annoyingly good-looking First Officer and Captain was just a bit older but annoyingly even better looking and ex RAF fighter jock. He agreed (with the UFO-not the looks) and reported to.....ATC...! We were told that there was no reported activity in the area or bordering control areas. Of course, everyone on 123.45 took the michael but us two Hollywood good-looking dudes were convinced. Of course first thoughts were of WX balloons but others on freq who had seen WX balloons confirmed that their sightings had not matched our observation.
Even though I have only one observation, in the 40 years, I heard of many from very credible sources. Quite a few were ex-Mill and very, very reluctant to bear witness. Cost me tons at the bar with - "Pleeze, pleeze, pleeze - I love all this stuff - !" -