I'd agree with Vic that it might depend on the age/type of flat. I can see a Soho flat of that period (is the story set in Pros era?) being the sort where all the mail would be dumped on a central shelf or table in the lobby/hall for collection. In the real heart of Soho there's a fair number of flats above shops/restaurants rather than blocks. Bigger, more modern places would have the named postboxy things but I wouldn't really associate that with 70s/80s Soho. Great place to set a story (or part of one!) :D