heater problem?

fiesta zetec 1.4, 58 plate, heater knob for directing the flow, into car or screen etc, doesnt work properly anymore,is this an easy fix,or do i have to have a new heater box like my garage has said, 1200 pounds seems steep for a knob,its like its just come off at the back,can anyone help please,

dave ramsey
April 2012