Thanks lv100 - I appreciate you coming back to me - I managed to resolve the issue.
Here's what I did incase it resolves the problem for anyone else.
The problem came about after putting an MP3 CD into the head unit (which I now know it can't read
The battery also died somewhere in the process here and after fitting a new battery, the Multi changer wouldn't recognise the cd player or gave the CD ERR or Link ERR message.
To resolve the, I disconnected the battery for 15 secs (the good old reboot!) then selected any source other than the CD (other wise it automatically started to play the MP3 disc in position 1) and hit the EJECT button + 1 to eject the offending disc. Problem solved (Phew, since the replacement cost was looking like £800).