The CDs are rated for 80 min or 700MB. I have not previously looked at file size, only time. The file size varies from 650 ish to 808 but most of them were burnt to CD with ImgBurn - it's only the last few which have thrown up the error. I can now see why the problem arises but now can't understand why it hasn't mattered in the past!!!
I don't really know but I'm guessing that it was the size of the files.
Sorry for the confusion - the files take about 10 mins to burn - it's the audio tracks that are 65 to 75 mins when played!! As the CDs are rated at 80 mins, I assumed there shouldn't be a problem. Once you'd pointed out the file size was important it became clear that that could be where the problem lay.