I think one problem might be that I'm using 64-bit WIN10 (successfully with Finale 2009) and trying to load 32-bit Dolet 7, It's a non-starter. For the moment, thanks to Michel and other who tried...
Thanks, Michel, I haven't checked for versions of MusicXML - but it's a good point - as I simply use files found on cpdl.org, which is a vast repository for (mostly) choral music, but I don't reall...
Many thanks for replying, even if it was all lost in the process! Perhaps there's a nugget you can offer that might help me get on the right track Adrian?
Using the "Smart Lyric Extensions" option is also my preferred way, and one can always click the handle on each extension thereafter to change its length, if necessary.