i spent 300 quid recently and had 5k flyers delivered and picked up over 30 regular customers from it plus a few one off jobs which got my money back plus some more,
i'd say find a local leaflet distribution company and get some flyers delivered, spend some money on some advertisements in local shops, parish magazines etc, get your van sign written if you dont already,
you could quite comfortably get an extra 1k of regular window cleaning work for under a grand just be smart with your money and do what works for you,
the best way i pick up new customers is get a worker to come with you on a saturday and get him/her to canvass houses around where you are already cleaning a house, they find someone who is interested and you go and quote and clean them there and then, its good because you meet the custy and give them a price and it compacts your work