I'm not quite sure how to search for the answer to this very specific question, relating to high-rise flats (or tower blocks) in London. My novel is set in 1975, although I don't know if that's important. My question is simple: how does a non-resident get into a tower block to visit a friend who is not expecting them? Do they press a button? How can the resident see who it is that wants to come up and see them?
If you want a picture of the type of tower block I'm imagining, this is a pretty good representation:
Great, I got exactly the answers I needed! Thank you so much, everyone.