Barristers Clerks, what powers do they wield in chambers?

I have been watching the TV drama on BBC called Silk. Billy, the barrister's clerk seems to wield a great deal of power in the Chambers. On the one hand he defers to the barristers by calling them sir and miss and yet somehow he also appears in control of what they do. What would be his position in the hierarchy of a chambers in real life?