2002 Vauxhall Vivaro display shows 'message'

After replacing the battery, I cannot get the radio to work. The information display shows the time and temperature as normal, but where the radio station is normally displayed, the word 'message' appears. I have previously input the radio code sucessfully, but now cannot finish the procedure - the code goes in fine, but as soon as I press the AS button to enter it, the display goes back to 'message'.
My big concern is that even with the ignition switched off, the display will not turn off. I have removed the fuse for the time being, so as not to drain the battery. Can anyone shed light on the 'message'? I guess my next course of action is to try to replace the stereo.