I clocked this one earlier and decided to keep an eye out for replies. They are thin on the ground, which suggests that it is a tricky one and will probably cause a bit of head scratching and maybe even become a headache.
Back and sides are clearly straight forward, but the balconies and panels would be a pain.
The two things that spring to mind are (1) that it will be almost impossible to have have access to all the flats on the same day, even if you get it now and again, can they guarantee a regular monthly performance. (2) can you open both patio doors to access the panels. My guess would be that only one panel will slide leaving a hard clean for the second panel.
Whatever price you put in make sure it covers internal cleaning for the communal entrance and stair windows.
Set aside extra time for each clean, because if you need to go into each flat they will from time to time ask you to do inside, especially the patio doors. Be ready with a price or you will get lumped with, "can you just do this while you are here"
All in all it looks like a headache waiting to happen.
I would put in for £210 pcm , on the condition that it was a reliable monthly clean, any flats not open don't get cleaned but still pay. £350 quarterly. I would hope that would put them off.