How can I fix the radio/cassette on my Y reg Corsa after power interrupted?

I recently had the alternator on my Y reg 1.7 Diesel Corsa replaced, but when I picked it up from the garage the radio display simply said "SAFE" below the time and temperature when I tried to turn the radio on. I followed the instructions in the manual and, since I could not at the time find the radio code simply entered 4 random numbers a couple of times which obviously did not work, and then set about looking for the code, which I found tucked inside the glovebox. When I repeated the procedure, instead of
reading "CODE - - - -" after the beep it simply reads "7 SAFE" and I can't get it to change. Any suggestions? Should I try interrupting the power supply again and having another try?