Yes the cartridge works, in fact yesterday I changed the placing of the CDs.
I'm back in the front of the pc just now. The weird thing is that this morning on my way to worked the CDs played, and on my way back home just now it worked again so it seems like the problem solved itself.
However I don't want it to happen again so I tried thinking what could have caused it and came up with 3 possible causes which I'd like to discuss with anyone who may have experience of such problems:
 HEAT - the car remains out in the sun all day 7:30am to 3pm. The sun here in Malta is really hot
 GENERAL ELECTRICAL MALFUNCTION - The right rear light needs servicing according to the dash electronics. In fact in idling the light varies in intensity. (Car's booked for a service anyhow).
 CHANGER MALFUNCTION - the changer itself or laser could malfunctioned.
The first one is most plausible since the car cooled overnight. I can't imagine how the second and third could have solved themselves.