How do I reinforce the cavity in a plasterboard wall so that I can hang my flat screen TV on it?

As the cavity is only 2 inches you may have to go through to the brick wall and fix something like corkscrew cavity fixings, the only other option i can think of this very second is putting a piece of timber in between the studs by making a small fist sized hole in the plasterboard slide your timber up through the hole and pull back on it so you can fix screws into it to hold it in place.