Probably, you have to install it using imei-hack. I had the same problem with a vodafone uk mobile.
Be sure that you make the copy of the imei correctly because a mistake at this point would be FATAL (5.img into IMEI-BACKUP).
Once you finish the tutorial, check that the imei is correct by clicking *#06#.
EDIT: Another topic: http://www.modaco.com/index.php?/topic/362677-How-to-downgrade-from-B952-to-GB