Disco 1 stereo coming up with security

Hi guys, I've just fitted a new battery to my disco it's a 300 tdi 98 manual, it has a standard land rover cassette player, it asked me for a code, I typed it in, I think it is the right code, I pressed BND button in whilst pressing the volume button in to turn it on, it worked for 3mins then came up with security, I repeated the buttons I pressed then it came on again, I had to re tune the radio stations in this time, it worked again then went off after 3 mins, it was beeping as well, not loud very faint, it went off again and said security. I did this 4/5 times. Any ideas guys?

Pretty straight forward if you have tha basic head unit wit a cassette player, if the incorrect code ha been entered just press the BND button for approximately three seconds and CODE IN will be shown in the display. Just enter the correct code and that's it. No other buttons including the volume need to be pressed.
If the unit is loosing its electrical supply intermittently then you will have to enter the code again when power is restored.
Code entering with the cassette head unit that a multi CD player is connected to it, is slightly more involved, but the power connections issue still applies.