Any question answered...

http://www.protape.co.uk/ is what you are thinking of.

So are you talking about red book - the standard definition of audio cds - or is it just the media that makes the difference with those old players?

^^^ Protape, indeed.

It's the media that makes the difference, when data CDs came onto the market, people were competing to squeeze as much data on as they possibly could. When you write an audio (as in, red book - of course, only red book audio format will work on an old player) CD to one of these data CD-Rs the header data (to do with how many tracks there are and where they start on the disc) can end up right at the very edge of the disc, beyond where an older CD player was originally designed to read. Thus, they don't work.

At least, that's my understanding of it.

cheers both.
i'd dismissed the labelling of cd's for 'audio use' as marketing before now but will give it a go with one.