Sorry I should have answered your question also.
As above really is the way forward. Give it a good soak with an APC, get a brush into it and agitate. A good rinse and you are all set. If you use APC for this don't forget to use it everywhere else while you are at it.
It's great for cleaning tyres, plastics and rubber seals. Just be sure to wash it again afterwards as it will run down the paint with dirt leaving marks.
Lee.