Hi , oven cleaning can be hit and miss, depends on which part of country your in IMHO there are already too many with more starting up all the time all sorts of tanks available some good some so/so lots of people think it's get rich and just see pounds signs ,but be warned its not always, customers on average repeat 12 and 18months with a few may do it 6 monthly
Ideally you need someone ( ya misses) to book the jobs and ring round for you when your out cleaning for repeats and contract work, if you can why not do end of tenacy cleans as well ,also kitchens and extractors(commercial)
Notice the ones selling tanks wouldn't like to sell to many in there own area as they would fall in to the wrong hands and people would work for £30 an oven because there are to many already which pushes price down just like carpet cleaning
Go for upmarket customers be quality ,dress smart ,be professional and charge a fair reasonable rate don't be to cheap(ok when your. Learning the ropes)
But cheap ent always best that's why folks shop at waitrose and marks n sparks rather than aldi and the like