CCRT700 cd safe?

Hi all,
I've got a cd multichanger in the boot with cassette unit up front. I've put the code in for the radio, all ok.
When I try to input the cd code it doesn't allow me.
Done so far:
press cd
turn unit off
Hold cd button and turn unit on

At this point it should beep, but doesn't

On the display it shows "cd code"

I've waited 40mins with ignition on and nothing changes.

It doesn't give me anyway of inputting the code.

Any idea would be gratefully received :)

Ps. It worked fine before battery change and Vx dealer had no idea!