Ok, I have tested two more videos. Each are .m4v over 27 minutes long.
Neither would save as a disc image, they reached 99% encoded then failed with the -111 error. I trimmed each in iMovie to 23m 17s and saved them as .mp4 before attempting again. Each saved as a disc image successfully. I then trimmed the original files to 23m 18s and tried again. The videos did not save, -111 error. So it seems that Toast cannot burn a video over 23m 17s long.
I'll inform Corel/Roxio of this and see what they say, if anything.
If you haven't already could all that are having this problem raise a support ticket as the more people that report this problem the more seriously they'll take it, thanks.