Who is Peter Guilliam from Tinker, Tailor, Soldier, Spy and what is their importance?

Peter Guilliam is the son of a French businessman, who spied for the British during WWII. His mother worked with codes. Peter works for the Circus as head of the scalp-hunters. He had worked in French North Africa until his cover was blown eight years earlier. It is Guilliam who brought Smiley to Lacon's residence where he was told about the mole, and it is Guilliam who obtains the documents and information that Smiley needs from inside the Circus.