Who is Toby Esterhase from Tinker, Tailor, Soldier, Spy and what is their importance?

Toby Esterhase works at the Circus. He is in charge of the group known as the Lamplighters, which provides support services for safe-houses and surveillance. Esterhase was recruited by Smiley in Vienna, and he is one of the four men that is identified by Control as being involved in Operation Witchcraft.