After much experimentation, some thoughts:
Editing a recording in eyetv clearly upsets the relationship between eyetv and toast. the way around it is not to edit, or to export the edited file. the best export format i find is h.264 (although i did finally have success with Elementary Streams, as advised by tsantee). h.264 appears to limit the resultant file size while keeping quality acceptable. toast detests trying to write mixed file types. i also find the most stable way to write the exported file to dvd is to drag from the Movies folder in Finder (for macs) direct to toast, rather than via eyetv.
there are clearly some glitches in eyetv and toast in terms of the file sizes displayed in various windows (some are vastly optimistic, i suspect). there are clearly some problems in the menage a trois between the OSX Finder, various internal and external dvd writers, and various types of blank media. there are also clearly some problems between Finder, writer, media and disc utility as well.
thanks to whaletosh and tsantee.