Mine was very similar to that, just a buildup of airborne grease and crap.
I made a solution according to this page:
http://astrosurf.com/luxorion/reports-cleaning.htm
and it took about 5 goes to slowly and gently remove the film of crudola.
You can leave the corrector plate on the scope, the very very edge probably doesn't matter that much. Mine ended up perfectly clean again. I also tried the Bintel solution, but it didn't work well for me.
I ordered a bottle of Craig Chemicals 99.9% Isopropyl alcohol from my local chemist. They got it in, it took a few days. I don't trust the one from Bunnings, but it is probably fine.