I used Gtechniq C4 on my scuttle, grilles, wing mirror bases, window rubbers, windscreen washer arms, mudflaps and anything else black plastic - wiped first with isopropyl alcohol to get it really clean, then a coat of the C4 brings it up good as new.
Dunno how to get that bumper plastic off - let us know the method if you figure it out; I need a new one due to a nasty scratch on mine.