Get a load of leaflets and get knocking on some doors in the evening mate (between 6-8) and leaflet the houses that you get no answer
Do this for a few weeks, mon-fri, and you'll have the amount you want in no time
("Hi, just a local window cleaner in the area. Just wondered if you might be looking for a regular window cleaner at all".... Dont waste your time with any more spiel than that, you're just looking for easy yes's at the moment)
Also get a hoody/jumper printed (if you haven't got a business name yet, just get 'Window Cleaner' Printed on them so people can see it)
Do your new jobs on a saturday if you can so you've got a decent chance of people seeing you around cleaning
If you're only looking at £150 a month at the moment it shouldn't take long at all if you do all that...