I have just sanded my corian counter tops after nine years use, and scratches came out nicely but it left a satin dull finish( I used 330 wet/dry) and some sander marks. Is there some polishing I need to do to the surface if so with what?

Yes, there are polishes designed for post-sanding corian.

Lowe's and HomeDepot may carry them. If not, here are some online sources:
http://froogle.google.com/froogle?q=...en&btnG=Search