One suggestion was to smear AutoSolve or similar on a strip of brown wrapping paper and pull either end alternatively. I tried it and it seem to do OK, then I got bored and went for a ride instead.
Brush wonder wheel alloy wheel cleaner on leave about 1 min then wipe with a cloth exhausts return to original colour and then wash off with water repeat if required then final buff up with autosol. Pipes then when bike run gradually turn a lovely gold colour.
On another forum about another bike, someone swore blind by Lidl's (or it might be Aldi - these Germans all look the same to me ) stainless sink cleaner.
"Wonder Wheels", used for cleaning car alloys, works a treat followed by Auto Glym metal polisher - just make sure you don't splash it on any painted bits!