This is actually a tip I was introduced to by one of my cleaners. Try mixing bicarbonate of soda with water, you should get sort of paste, which you can spread over the oven with a cloth. Leave it for 30 minutes, so that is soaks in and soften all the marks and other dirt. Then you can either use one of those spongy oven scrapers or just a regular wallpaper scraper. Be careful though, it's easy to scratch the oven with a wallpaper scraper. I use a cheap plastic one I've found in a hardware store somewhere in Central London. Hope it helps!