The £75 service refers to the head unit (Radio) I think not the CD changer.
Does it make any noises inside when you press the eject button?..if not, check that youare getting 12V. to it.
If does sound as though it's trying to eject, probably there is a disc jammed inside, remove the unit, take the cover off and all will be revealed, the disc may be warped, it usually happens when "Naughty" burned copies are used. You may scratch (Wreck) the CD in removing it.