Looking for a London office space

I need to rent a writing office in Central London — I'm looking for a room of my own, around 10 feet square (plus or minus), with a door I can shut. Unfurnished is fine — so is a space without broadband, provided you don't mind my getting a line installed. East end strongly preferred — Brick Lane, Old Street, Hoxton, Liverpool Street, Moorgate, and environs. An ideal situation would be renting a private office in a larger office on an annual lease (I'm not interested in an open plan space or in sharing a room with another person), provided that I could have 24 hour/7 day access. Rate negotiable. If you know of a place that meets this description, drop me a mail!