Best stuff I've found is to soak it in washing soda or biological stuff, and then scrub it with a green scourer and Astonish paste.
We used to use Ajax or Vim (the stuff used to scrub the steps) but the Astonish paste makes life easy, and no bleach splashes as you scrub.
We had a speaker at the Young Wives years ago, and he was advertising/selling Carbosolve. I mind it was surprisingly expensive and I didn't buy it, but he claimed that it would even de-gunk car exhausts
Don't know if it's still available, but might be worth a looksee. He demonstrated it on a well used chip pan, and it did come up shiny and looking new. Neat trick that for an old chip pan, which usually ended up cruded and caked with burnt on fat on the outside if not cleaned frequently. Many folks never bothered and the layers just grew thicker and thicker, a bit like an overdone coating on cast iron, iimmc.
M
p.s.
I've had a google, and lo and behold, the carbosolv stuff comes up as Fat tank cleaner.....
http://www.fattankinternational.com