I agree with Colin on this one,
oven cleaning is a good starting point, I started off with a low budget 4k, used oven cleaning for 18 months part time to raise enough for really good equipment for my commercial kitchen / extraction system cleaning business which was the main aim, would not have survived without the full time job tho, I am by nop means alan sugar but my best advice would be, research, research and then some more research, know your target market and also your local competition. It is always handy to have more than one string to your bow.
Good luck, don't get disheartened you are entering a cut throat market where the customer expects to pay peanuts and has no Idea of the costs involved in setting up a cleaning Co.
MCM