Go up to the intercoms and press number 1.
Smile, drop your diaphragm (you sound more friendly and relaxed) and say " Excuse me calling, I'm Lord of the Rungs* the local window cleaner, how do I contact the management committee for this group of apartments?"
Repeat on all the numbers until you get a sensible reply informing you of where to contact. If the chairman/lady lives there then immediately press their button (ooh err missus) and say.
" I was given your name by (number of person you just spoke to) as the person I need to speak to to offer my w/c services - we do lots of apartments like yours - how do I go about quoting for you?"
Follow their instructions.
*If your name is not Lord of the Rungs then give that instead!