Yarlington resident here. A couple of months ago a van turned up and the workers inside were cleaning all of the outsides of the windows of the Yarlington properties on the estate. When I expressed surprise at their reason for coming they told me I should have had a letter about it, I hadn't, they told me it seemed no one else they had spoken to in the area had had one either. Needless to say, I wasn't going to refuse to let them clean the windows free of charge, they had Yarlington vans and IDs. Turned out the day before a friends flat had been done while she was at work, she'd not had a letter about it either.
Don't know how many councils\housing associations do this. For our house it might have been because the previous tenant was old and needed help - alarms, big switches, easy to use taps etc, so had help with windows and the details for that help might have not been changed. Last time we rented from council was 15 years ago and it was your own responsibility then, though.
It is always worth asking the people who own your flat if they provide a window washing service. They might provide a free service, a subsidised service for their residents, or none at all. If you don't ask you will not find out.
But paying to have it done by a window cleaner yourself doesn't have to be expensive, ask neighbours who does theirs and if they would recommend them, look in the local phone book or look on-line, someone should turn up. Ask for prices first before someone comes out, but a first clean of really dirty windows might cost more than subsequent cleans because it will take longer to do.