Cheers for the replies.
On the rattle issue I am having new stainless downpipes fitted next week, if that doesn't cure it new stainless polished manifolds will be the order of the day if they don't cure it I will hang myself. It's def not a heat sheild I've had at least 15 mechs take a look at it over the last 8-9 days and all of them are confident that it's a heat shield, come out with a gigantic screwdriver and start poking and prodding around then say "maybe it's not a heat sheild then". At one stage I had a mech standing under the center box, one under the CAT, and one under the engine/downpipes - each of them said the rattle was coming from their allocated part of the system!!!!! Anyway due to the replacement of the entire system from the downpipes back it cannot emit from there.
On the idle problem I don't know what my next move will be yet.