Adelphi

A Time Lord messenger came to Earth to warn the newly exiled Third Doctor about events.

The Time Lord first met the Third Doctor when he appeared in his TARDIS while the Doctor was with his companion Liz Shaw. (AUDIO: Shadow of the Past)

The Doctor next met him when he was sent by the Time Lords to tell the Doctor that the Master had arrived on Earth. (TV: Terror of the Autons)

He was sent to see the Doctor again when he was with Sarah Jane Smith in order to persuade the Doctor not to travel with a future companion who the Doctor had not yet taken onboard the TARDIS. (PROSE: An Overture Too Early)

Appearance
He was dressed as an English businessman, complete with bowler hat and umbrella. His disguise was somewhat compromised by his first being seen to hover in mid-air, due to his co-ordinates having "slipped". (TV: Terror of the Autons)

His appearance and his exit were accompanied by a sound reminiscent of the TARDIS, implying that that the Time Lord messenger travelled in one which Chameleon circuit made it appear to be invisible.