Became a maintenance instructor in 1966 anbd attended DC9-10 series school. Douglas was very aware of the problem. The DC9 did not have a "elevator boost control". Instead there was a "disagreement mechanism" so if the piloit was pushing the column forward but the elevators wer "UP", the hydraulic actuator operated and pushed the elevators "DOWN". In any noprmal flight configuration, the flying tabs did the work and the hydraulic actuator just went along for the ride.
Did the British authorities requie a "stick pusher" for the DC9?
DC9 had also installed "vortillon fences" under the inboard bottom of the wing and strakes up forward on the fuselage, all intended to aleviate deep stall tendencies.
I actually saw the TIA DC8 at JFK that had jammed elevators due to some tarmac paving breaking up and pieces flying up and lodging in the gap between stabilizer and elevator. I heard loud engine stalls and looked out the side window of a B727 I was troubleshooting. Saw the aircraft in an almost vertical position aand then fall off and go right down. Giant eruption of flame and smoke followed. Ghastly sight.