Does the whole gas hob need replacing?

I own a flat that is managed by an agent. The agent says there was an emergency call last night because the tenant couldn't turn off the gas hob. They have advised that the whole hob needs removing and replacing. I've done some quick reading and it seems that the knob could just be replaced. (I often think their work men try to earn more money than in necessary when they repair things so I'm wary!)