Get your car tested for speedo accuracy.... It may be majorly out... I'm not sure if when in court you can request the radar be tested for accuracy, usually they are spot on (as in perfect)......
Did you ask to see the radar? I believe you can... and they can't book you without evidence... and to be doing EXACTLY 140km/h is wierd... i'd guess 139, 141, etc not spot on, sounds like he was guessing TBH...
Aaron