EPISODE: The Armageddon Factor Part Five
OVERALL EPISODE NUMBER: 504
STORY NUMBER: 103
TRANSMITTED: 17 February 1979
WRITER: Bob Baker & Dave Martin
DIRECTOR: Michael Hayes
SCRIPT EDITOR: Anthony Read
PRODUCER: Graham Williams
FORMAT: DVD: Doctor Who - The Key to Time Box Set (Ribos Operation/Pirate Planet/Stones of Blood/Androids of Tara/Power of Kroll/The Armageddon Factor)
Leaving the Tardis the Doctor becomes separated from Romana & Astra. Astra takes Romana to the Shadow while the Doctor, following the computer distress beacon K-9 detected, find his old classmate Drax, who was forced to create Mentalis by the Shadow. They start work on a device using Drax's damaged dimensional stabiliser. K-9 is sent to demand Romana's life in return for the five segments of the Key to Time. Drax is set to work freeing K-9 from The Shadow's control. The Doctor bargains with the Shadow, who says he has the sixth segment, and goes to fetch the Key segments from the Tardis but as he opens the Tardis door Drax blasts him with the gun they've made
At this point five year old Philip rejoins the story recalling the Doctor's college friend Drax and the Doctor crawling around tunnels, throwing the controlled K-9 to him. Drax as a character had been floating around Bob Baker & Dave Martin scripts for a while, first appearing in early drafts of The Hand of Fear, as a slightly shady Time Lord Mechanic.
Drax is played by Barry Jackson, who was previously Ascaris in The Romans and Jeff Garvey in Mission to the Unknown. Nowadays Barry Jackson is best known as Dr George Bullard in Midsomer Murders where he appeared in nearly every episode from the show's inception till his departure in 2011. In a few of the earlier episodes that George Bullard doesn't feature in his place is taken by Dr. Dan Peterson played by Toby Jones, later to memorably play The Dream Lord in the 2010 Doctor Who episode Amy's Choice.
If IMDB had a tool similar to Cricinfo's Statsguru I'd try telling you how many other Doctor Who actors have been in Midsomer Murders. It hasn't so I shan't!