P2 is correct, what needs doing should be listed in your handbook.
Expect to pay anywhere from £200 - £350, very much dependant on where you go. If going to a main dealer look at the higher figure of the two, and they will supply everything. Idle does seem a bit high, usually only does that when cold but then the mornings have been a bit nippy of late. Signs of exhaust blowing are really as simple as visible gasses escaping (check the flexi joint on the Y-pipe and the connections on the backbox) and a bit of extra noise.