May help.
The company was BOAC, I thought the incident was out of Dorval, Montreal (but could be Toronto), probably a -436.
The conversion course we did together was in 1978 and I had the impression the incident was some years previous.
The chap was a captain on the 707 by then, so, if in the incident he was a P2, it possibly happened in the 60s.
I joined BOAC in 1970 and don't recall it happening in my time, but that may be because I was on the VC10.
I know that D P Davies, of AAIB fame, was closely involved in the investigation (my acquaintance was not very enamoured with him).
He stated that the trim even ran away with the brake on (wasthere a brake?) and his hand trying to stop it.
Never flew the 707 but I think it may have been something to do with the servo tab working in the wrong direction.
finncapt, I also joined BOAC in 1970 (as a young Second Officer) and started on the 707-436 and I recall this incident being talked about. Your version of events concurs with mine and I believe the aircraft had departed from Montreal. The cloudbase was low and radar assisted then to avoid higher terrain as they had initially reduced power to maintain pitch control. Despite the fact that the stab trim brake was on the trim continued to run away slowly and I believe the F/E put both hands on either side of the wheels to prevent further runaway. I believe the cause of the runaway stab was incorrect grease being used on at least one of the clutches on the stab trim control.