it was a clear dry sunny evening with good visibility, temperature +16c. i noticed a large aircraft contrail heading toward my location at high altitude. i used binoculars to view the overflying aircraft which was a 747. the aircraft was a confirmed on radar as a cargolux 747 freighter, flight #clx775 lx-wcv b74f, heading 160 degrees at altitude 33,000ft. whilst viewing this aircraft through binoculars i noticed a metallic silver sphere shaped object which was reflecting the sun, this object passed in the opposite direction to the 747 (heading approx 360 degrees). although the object appeared lower than the 747 i could not ascertain if was actually beneath or higher and further away, also the object was small and not visible to the naked eye. the object appeared no larger than a 747 engine. the object appeared to be travelling at a much slower rate than the 747 and was moving steadily with no course deviations and had no visible contrail or sound. i viewed the object for around 60 seconds before it appeared to either climb or change course in the direction of 020degrees. at this point with the object still in view i decided to fetch my camera with telephoto zoom lens, but when i returned with the camera i was unable to relocate the object even using binoculars. my radar was showing no ads-b/mode-s/mlat aircraft signals in the area other than the 747. the area in which the object was sighted lies within a main civil airspace corridor, military traffic crosses this corridor 7 miles to the north in a w-e and e-w direction usually at 14,000ft. my initial thought on seeing the object was that it was a spy plane or uav, which i soon dismissed. the object did not have wings or a tail plane and was not flat like the b2 spirit aircraft. i would be interested to hear if you have any thoughts on the identification of this object.