1957 Farnborough Air Show featured 90 Valiants and Canberras of Bomber Command contrailing at 35,000 ft as here:
Original photo appeared in 13th September 1957 Flight Magazine and can be seen here:
bomber command | pdf archive | aviation traders | 1957 | 1341 | Flight Archive
As quoted in the article:
....After civil transports, it was the turn of the R.A.F., and today there was something splendidly new as a background to the formations. Approaching high from the east were the contrails of the first dozen or more of over 90 Valiants and Canberras of Bomber Command which, flying at 35,000ft, were using Farnborough as a “target for operational exercises.” While the C.F.S. Provosts manoeuvred with precision; while the Javelins’ noise was felt in our stomachs; and while the glistening black fish, roundel-eyed, that were the incomparable Hunters of 111, swam in rock steady shoal around the lower sky, the high trails formed a fantastic backdrop. To the east, a tangle of thin white threads; above and to the west, a strong sunlit pattern of bold, white brush strokes on a glaring blue canvas.
This was display spectacle unsurpassed for many years. It is much to the credit of the remaining pilots and aircraft that no sense of anti-climax followed....
I was abroad in the Far East in 1957 so missed the 1957 show, however I'm fairly sure I DID see a similar contrailing Bomber Command demo at Farnborough, but cannot recall the date.