I believe that the BAe flight-test crew were engaged in high-speed flutter trials with the first K2 tanker conversion ZA141 (formerly G-ARVG) . The aircraft was oversped in a dive and the crew deployed full speedbrake. This, of course, moved the centre-of-pressure rapidly aft and pitched the nose even further down. The pull-out from the resulting dive overstressed the aircraft and the fin broke at a previously undetected crack. The tail was removed and replaced with that from the RAE Bedford aircraft XX914 (formerly G-ATDJ).
As a result of this incident RAF VC10s were fitted with a baulk to physically prevent full speedbrake deployment above 15,000 feet. This device is a simple hinged plate that the crew fold into postion during the climb checks. It can be removed to allow full speedbrake deployment below 15,000 ft and is confirmed as "out" during the landing checks.
Observation from the periscope also shows how much of pounding the tail structure takes as the aircraft is taking on fuel inflight.