Oven cleaning is not rocket science, very little cleaning is! We have looked at adding it to our ever growing list of services, however I'm quite happy subing out the work to our local lads as it can be a messy job.
I know the lads who we use basically take the trays outside and put them into a hot caustic tank to soak, then they use a window cleaning scraper to remove the heavy deposits from the base and side and finished off using Astonish Oven Paste. The end result is always perfection.
The caustic tank has a heater on it that they plug into a mains supply, but by what they say, they fill the tank and heat it up before they leave for their first job and it stays hot enough for all day. I think they do about 5 a day @ £75.00 a shot for a standard oven, hob and extractor.
The caustic tanks are the most expensive parts to buy, need to be best grade stainless steel with a sealable lid to prevent splashing when driving.