Is it on the main road?
Please tell me you are not a postman, I`d hate to be on your round! To be honest it`s pretty easy to find, if you are on Berwick Street walk towards Oxford St and it`s about halfway down on the left, the door will be open as mentioned, and if you peek inside you`ll see a paper sign on the wall saying "MODEL".
Just walk up the stairs (hence `walk ups`) and there will be a girl on the first floor, and another on the 2nd, it`s pretty discrete during the day too, which is a bonus.