Fitting folding doors to a cavity wall.

Next weekend I'm fitting some folding doors to an external cavity wall in place of an existing window. The fitting instructions say "A cavity fixed jamb must be firmly strapped to ensure no side to side or rocking movement."

Does this mean that I'll need to strap it with something? Will screwing it to the external bricks not be enough?

Thanks in advance.

I assume that the instructions would be for fitting into new work otherwise you would have to hack the plaster of the reveals, if i was fitting i would fix straight to the brickwork.