Autosol gets my vote. Around £6, and can be used on anything (I've used it on alloys, nuts, bolts, badges...)
When I picked mine up a few weeks ago, the exhausts had clearly never been cleaned. They looked bronze. After applying nearly an entire tube of the stuff, they've come up like new, and actually look silver/chrome again.
I didn't get any 'after' close ups, but they really are like new.
BEFORE;
AFTER;