It should be relocated at Second Time Lord (The War Games) because The Legacy of Gallifrey identifies the First Time Lord in TWG as Goth and the Third Time Lord as Socra, the latter being explicitly a different person from the messenger in Terror of the Autons. By process of elimination, if he was also part of the Trial, this makes the Messenger the Second Time Lord (a page which, also on the faith of Legacy of Gallifrey, should be retitled Adelphi).
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He was a member of the tribunal who sentenced the Second Doctor to exile on Earth, (TV: The War Games; PROSE: Doctor Who and the Terror of the Autons) and also was a member of the High Council. He had been sent to the tribunal on orders of Lord President Pandad IV as a direct result of Rassilon's agent, Socra, being asked by his Matrix Lord masters to keep an eye on the Doctor. (PROSE: The Legacy of Gallifrey)
After the Doctor's defeat of Helios, the messenger spoke to the wardens of Shada, requesting that they release the Master and keep the Doctor busy on Earth. (PROSE: Prisoners of the Sun) He then travelled to Earth to warn the Doctor of the Master's arrival and of an impending trap that the Master had set for the Doctor. When the Doctor asked for assistance in dealing with the trap, however, the messenger simply disappeared. (TV: Terror of the Autons)
He was dressed as an English businessman, complete with bowler hat and umbrella. He compared his appearance to that of the Doctor's, stating that some Time Lords chose to remain inconspicuous. His disguise was somewhat compromised, however, by his first being seen to hover in mid-air, due to his co-ordinates having "slipped". (TV: Terror of the Autons)