I was piloting a light aircraft, and I chased a round flying object that moved at very high speed

A friend and myself,both qualified commercial pilots, were flying a Grumman Cheetah from Bournemouth,in Dorset,U.K.,to our home airfield at Elstree,just north of London. Approximately 10 miles South West of Watford we saw a silver ball approaching us at high speed.It went straight past us, and stopped dead, with no deceleration. We turned to follow it, and it seemed as though it was playing games with us,rising vertically at high speed, and constantly changing course.I know of no technology that could duplicate the way this machine moved.We never discussed the incident afterwards,as we did not wish to make fools of ourselves,particularly as we would both be looking for employment as professional pilots soon.
Date is approximate.  We would like to invite the witness to determine the exact date from his flight logs, but he does not provide a return e-address, just a URL for his business, which does not seem to be current.  We would very much like to correspond with the witness.  PD