Alarm bells should be ringing if house is only one in street minging. it's rented accomodation, it's a flat, customer is rude, you've got a gut feeling.
I charge double for dirty initial wash after explaining I don't do 'end of tenancy' cleans, etc, etc. If I lose a customer after initial wash, at least I've recovered my cost of labour for extra time spent cleaning, so no loss of earning.