Thanks very much for the quick replies. I have tried slowing the burning speed down to 16x and it doesn't make any difference to the playback compatibility. I also tried a Sony Optiarc drive, and it was marginally better than the Pioneer DVR-A18, but still not as good as the old Pioneer DVR-112.
Another factor is the blank discs we use. We use bulk-packed anonymous blue-topped CD-Rs, which are manufactured by Sony according to Nero. I also tried some branded Sony CD-Rs, and strangely the branded Sony discs make better copies than the unbranded ones.
I tried connecting the Pioneer DVR-A18 drives to my computer instead of using them in the copier, and one drive appears to be faulty: it comes up with 'Power Calibration Error' when starting to burn, but the other one doesn't. They were bought at the same time, are brand new and have the same firmware and I was using the branded Sony discs in both.
I'm coming to the conclusion that although the old DVR-112 drives are undoubtedly better than the new DVR-A18, the DVR-A18 does make acceptable copies. The playback problems are in older CD players that may be past their best. It was just odd that the old CD players had no trouble with the copies burned on the old drives.
I'm having to tell people that they need to buy new CD players because we have got new burners in the copier, and it doesn't seem to make sense, and doesn't go down too well either!
Thanks again for the advice,