crank her up, let it run
get your head under the passenger footwell and stick your arm up into the far left corner, upward, under the dash. a torch will be needed, and a weird bendy arm with fingers that can bend backwards.
Nearly did my back in doing this shit. once you find the box you can slide it out of its holster pretty easily making it easy to disconnect it.
its awkward to get at, you'll probably not notice it at first if you've a bunch of wiring blocking your view.
central locking box and alarm box(if you have one) are under the dash at the passenger side too but very easy to get at compared to this wee baby.
once done turn engine off, turn back on again to check, key light should be out on the display also.
pricewise i'd check ebay, see what people are selling them at.