Grr! I've just spent more than half a fine sunny day trying to work out why the 'Mac OS error result code=-50' message kept appearing when trying to burn a Film 4 offering to DVD, so it's been very useful discovering that it is a persistent problem for others. Thanks everyone for the postings. Maybe someone should tell Roxann! On one of my many tests I used a Film 4 advert, only to find that it suffered in just the same way; and it was only subsequently, when employing some footage from a BBC 1 outside broadcast that I began to realise that the problem was probably localised.
The only other problem I have ever encountered in trying to burn a broadcast TV programme to DVD with Toast 7.1 was with a Channel 4 programme recently. No audio was present on playback. Annoying, but on that occasion I didn't care enough to begin tearing out my hair . . . However, it's good to have an explanation and the knowledge of how it might be overcome in the future. So thanks.