There's a guy who does all of our windows - he seems to work in the St Georges Rd area. He's very good, does either fortnightly or monthly visits. When I next see him (I think I owe him money) I'll see where else he cleans - what bit of SE1 are you?
If your windows are being made dirty by neighbouring building work why don't you have a word with the developer and see if they will pay to clean your windows. With schemes like the Considerate Developer scheme many developers have shown that they are very keen to retain good relations with the properties and individuals around their developments. I would be interested to hear what they say.