You will need process tank first of all which you connect upto a gas bottle and heat up, you use a high akaline solution in the tank, then you can take everything out that will come out and set to work on the oven, the process tank will soften the carbon and grease so it can be more eaily removed.
Prices vary from £45.00 / £50.00 for a single to £90.00 ish for a range.
I have a tank for sale and will let you have the instructions / pricing guides etc a copy of a leaflet that works if you buy the tank. I looking for about £400.00 if your intrested.