I don't know if BA have put anything in writing about room upgrdes, but BASSA definitely have. The only thing I could find on the BASSA website was something along the lines of 'Please fill in a report if you are operating as CSD and don't receive a room at the same grade as the First Officer(s)'.
What BA have put in writing (in every single hotel contract) has already been posted. BASSA believe that they have an agreement that CSDs should have rooms equivalent to FO's. Unfortunately:
i.BA deny the existence of any such agreement.
ii.BASSA cannot produce a written copy of said agreement.
iii.No minutes exist of the meeting where said agreement was reached.
Which would lead most non-BASSA people to a fairly logical conclusion.