I like cleaning flats, even more so since converting to WFP a couple of years ago. Though they're only £6 per clean all the blocks I do have at least ten in them, and with them all together like that you can knock out ten in an hour easily enough, giving you a healthy hourly rate.
I know of a few window cleaners who don't like them, prefering instead to concentrate on bigger, higher paying customers. I do them too, along with a lot of contract work and carpet cleaning too, but for earning money quickly I don't think you can beat blocks of flats.
Out of interest most of these flats have two front windows and three back windows, and last year they were £5 each. I lost no customers raising the price by a pound so I guess they must be happy with my work.
Mind you though, if I can't clean a window properly after 17 years of trying I guess I'm in the wrong job
Good luck with your flats!