Sorry.. but i'm aussie.. so dont know this N Reg thingy...
What year model is it?
If its non valvetronic.. sounds like the Idle control valve. Rip it out and clean it.
(never helped with mine car.. amazing how an idle can not be related to the idle control valve in the slightest).
Yes... i'm a total car nut! Why else would I upgrade my mower to rev to twice its standard rpm!