Everyone has their own preferred method of cleaning the c5 bodyshell, so hopefully others will post some alternative suggestions here as well.
I tend to use Cif bathroom cleaner and a large paint brush when renovating C5's shells. To get the cleaning fluid into the "grain" of the plastic, the paint brush needs to be adpated by chopping the bristles down so that it becomes more like a scrubbing brush.
I've not used it myself, but I've heard you can use brake disc cleaner to get rid of some really ground in dirt. You really need to be very careful with such strong chemicals

. Another alternative is a bathroom cleaner called "astonish" which was in the cleaning kits sold via c5alive before I took over.
Some scratches can be smoothed out by using "wet and dry" sandpaper , but again care needs to be taken to avoid smoothing out the grainy finish.
The bodyshells can put up with quite a bit of abuse during the cleaning process !