James
People pay at different times. Some pay on the day, cash, (I like those), some pay at the end of the week, some pay at the end of the month when they get paid. This is window cleaning, this is the business you are now in, get used to it! If it really bothers you, then find a solution to the problem! If you don't like it, then get out and do something else.
As for 'Common' houses, in my opinion, these will be your best earner. You want small houses, semi detached, well kept, no conservatories, side gate, good parking and lots of them.
Good luck.