It was a chrome ball,approx 4'0" diameter.No markings.

I was completing a Commercial Pilots course.We were flying a Grumman AA5,at around 2000 feet altitude,from Bournemouth to Elstree.(United Kingdom)The UFO went past us at a very high speed,at a guess,500 mph.It stopped dead,and hovered,then started moving again.It could accelerate vertically as fast as horizontally.We chased it at various altitudes for about five minutes,before my co-pilot enquired what I would do if I caught it!At that point,I broke away,and the UFO then started to follow us for a few minutes.We landed safely at Elstree,and didn't mention this to anyone.Some years later,an RAF pilot mentioned that he had seen a similar UFO,and that it was a probe that monitored radio and TV.I had not initiated the conversation,and he was quite matter of fact about it.He also talked about mother ships,and that the government knows all about UFO's. This machine could accelerate immediately to astonishing speed,and stop in mid air instantly.I know of no man made aircraft that could do that.It was also very close to the Heathrow control zone,and no airforce would test something like that so close to a major civil aerodrome.I am sure that it was not of terrestial origin.

See above,or email me for the full story.The date is approximate,but can be given more accurately from my Flying logbook,which I do not have to hand.I wanted to fly commercially,and so kept quiet about this for many years.My companion can verify the story,if he can be found.I know his name,and that he was a qualified commercial pilot.