How do I get out of safe more on a Blaupunkt 300?

I drained the battery on my wife's Corsa by leaving the internal light on for 5 days and the radio/cassette. a Blaupunkt 300 now shows 'SAFE' on the dash display when turned on. I have tried holding the 'AS' key while turning the radio on, but this just causes the radio to bleep, and 'SAFE' still displays. I haven't entered any codes yet, so it shouldn't be locked out. Any ideas?

Dave