Looks like the Sierra Cosworth had rear discs with a diameter of 273mm. I've posted a link to some on Ebay but there are plenty of others supplying just the rear discs. it seems there are both vented and solid discs in this diameter. [Just did a search under "sierra disc"]
All of them are 4 stud fixing which means your initial thouhgts are correct - your rear discs have been adapted to 5 stud. (Presumably this is because you have granada fitment on the front)
http://cgi.ebay.co.uk/SIERRA-RS-COSW...QQcmdZViewItem