I've just had the trim off the exhaust to clean as it was looking well minging

, I tried using Cilit Bang powder as you can use it on chrome sinks & taps and its worked a treat

.
Soaked it in a bowl of hot water with the powder in and then used a wet tooth brush dipped in the powder on the tougher stains. It hasn't got off all the discoloring but looks a darn sight better than before.
Will post some pictures after I have been out a washed the rest of the car.