Originally Posted by
Beamr
I have no knowledge of this specific event, but in case of explaining a mach 2 target at 40k with a Harrier I'd google SR-71. Though it would be a bit slow and a tad low for the Blackbird
One of the military controllers at Northern Radar pointed out an SR71 to me one day in 1973. It would travel at 'normal' speed ie about 500kt then stop, then repeat. This was because they would climb vertically several times on departure and the blip stopping meant one of these vertical climbs (raw radar not MTI equipped.)
The same controller (same year) also pointed out a Phantom which was positioning to Leuchars fromm Coningsby. It headed up the Wash then turned north and the blips suddenly got wider and wider spaced as it went supersonic. I was told later they did brakes off to touchdown in 7 minutes.
Years later at Farnborough (late '90s) I often watched aircraft indicating FL660 appear from somewhere off the south coast heading just east of north and crossing high over Heathrow; I presume these were U2s.