Flats are very difficult to get into, most agents have window cleaners they know and trust , just the same as your loyal domestic customers.
The people cleaning them usually prioritise and look after them , so unless they mess up it is hard to get your foot in the door.
I got my first flats because the previous guys werent very good and there was constant complaints about them, so they contacted me and as far as i am aware i have only had one complaint and that was missing one window , which so happens was the biggest moaners on the block.