Always done my own.
Got myself a window cleaning kit off amazon (bucket, mop and squegee) and watched a few YouTube videos on technique.
Bit of washing up liquid in the bucket with some hot water does the trick nicely.
Clean the frames with a rag.
Before I moved down here and had a house. I figured out I was saving around £180 a year doing it myself.
In a ground floor flat now. We are supposed to get it as part of the service charges we pay but the "window cleaner" turns up once a month, signs his sheet in the communual area and buggers off. We now refuse to pay that portion of the service charges and I just do it myself.