Hi,
Buy a good hand held steamer, (I have a black and decker) one and a large plastic container at least 90cm long. Fill the container with boiling hot water at the premises and dip the racks and grates in. Search ebay for cleaning products.
The best tip I can offer though will cost you. Get trained by professionals. Ovens are intricate appliances and you will need training. Some do it for £200 a day, others charge a fortune but you WILL benefit from it.
Regards
Richard.