There is doubt about both the alleged overspeed and overstress - the 'Syerston' Vulcan had very probably already been substantially damaged by the aerobatics it had suffered at the hands of RAF pilots who had flown it at Hucknall.
When Avro's pilots flew aeros in the Vulcan, it was always given a thorough inspection afterwards. But this wasn't done at Hucknall in the same way - and the aircraft was enormously overpowered compared to the versions displayed at Farnborough.
One RAF pilot had previously looped the 'Syerston' Vulcan - no doubt seriously overstressing it as a result. Hardly surprising that the aircraft later disintegrated as it conducted the flypast at Syerston.