Peugeot PU 9833A R/Cass Woes?

Hi, Have just found this excellent forum. My problem is that my radio cassette is showing the word 'security' in the display. I have the code, but can't enter it. I've tried toleave it with the power on for over 2 hours as it says and the radio should reset itself, but it doesn't. I left it for 5 hours and still no joy. Does anyone know if there might be a series of buttons I can press to allow me to get the display to say 'code in' as it shoudl; then I can enter the code.

My thanks indeed