X Type 2004 radio unit not recognising the cd unit

I have a 2004 X Type with a cd changer unit in the boot. I had to leave the car for 6 weeks and had a flat battery when I returned. Once the battery was charged, everything worked except the cd player.

There is power to the cd changer as I can get the magazine out normally. The radio unit shows 'No Magazine' 

The radio unit shows 'CD1 NO DISC' when the cd button is pushed, yet the magazine is full of discs and there is power to the cd changer.

I have unplugged the leads and then plugged them back in. I have also disconnected the battery and then connected again in the hope it might 're boot' the system.

Nothing seems to work. Is there something I am missing or is there some combination of buttons to push on the radio unit to get it to recognise the cd changer is loaded with discs.

Any advice would be welcome,

Thanks