get some Meguiars Degreaser or Autoglym Engine cleaner.
cover electrical parts like alternator etc and then spray lots of the cleaner onto the other bits and use an old bursh and give it a good scrub. Leave for 3-4 minutes then use a hose and rinse off - don't linger too long on any parts.
dry off the excess water as best you can
then spray some Autoglym Bumper care on the plastic parts and leave. Job done