Walker (The Sea Devils)

Parliamentary Private Secretary Walker was a civil servant in the British Ministry. He was sent to the Royal Navy base (HMS Seaspite) on Fortress Island to deal with the Sea Devils. He ordered Captain John Hart to launch an attack on their base, just as the the Third Doctor was trying to negotiate peace with them. (TV: The Sea Devils)

Behind the scenes
In the novelisation, he is given the first name of Robert.