MANY thanks for the replies. Further investigations reveal that it's not the media (the disks).
I phoned the printers, who said that always happens at their end - they put our burned disk in and get no contents - they are now in the habit of putting it straight into an external drive (an LG DVD writer/reader) and proceed from there. They also commented that we are not the only customer with whom this happens.
Our of curiosity, I put one of those CDs into the Mac which created the burn, and it COULD read it ok.
However, IMO that doesn't mean that this Mac which I'm typing on, has a malfunctioning CD/DVD drive: only recently I put a music CD in there and successfully pulled off tracks for my iPod.
Hmmmm......
