How do I clean the roof of my "alleged" self cleaning FAN oven without using Mr Muscle products?

I have an oven which is supposed to self clean. This does not remove any dirt. I have managed to scrub the bottom & sides with elbow grease and scourers, but how can I clean the interior roof from all the fat splashes from Sunday roasts and roast potatoes? I tried Mr Muscle, which was fab, but after the 2nd use, it ruined the fan element. So I need something that will melt or dissolve the old dirt that is not going to damage the element. I called an oven cleaning company and they do not clean the roof, so that is no help. PLEASE HELP as cooking makes my kitchen smell!!!