I'm hoping to do C5 this weekend and I'm hoping to do it without taking the wheels off (S3 alloys are pretty open!) - did you take your wheels off?
Yes, as it was really the only way to ensure the whole wheel was fully cleaned.
My procedure was:
1) Wash wheels with Billberry Wheelcleaner and pressure washer (pw).
2) Loosen any stubborn dust by re-applying Billberry and agitating with an alloy wheel brush. Rinse with PW.
3) Jack up the car and take the wheel off (I placed the wheel on a workbench to make it easier)
4) Clean all surfaces of the wheel with Tar-X or Tardis (don't leave this to dwell for more than 5 minutes).
5) Rinse the wheel off with the PW.
6) Dry the wheel and apply Iron-X
7) Wipe down and rinse with PW.
8) Dry the wheel and clean with IPA (I used Halfords Panel Cleaner) to remove any remaining grease
9) Apply C5 - a little goes a long way. Use the applicator pads provided and be damn careful not to knock the bottle over!
10) Re-fit wheel - don't drive it for at least 6 hours to allow the C5 to cure.
11) Sit back and admire your shiny clean wheels, you'll notice water beads straight off once fully cured (a couple of days in my experience).