I found the answer, based on what you said about Toast recognising 16:9 video. Final Cut Express was not exporting the QT movie in 16:9 format because I had used the simpler Export > QT Movie. Instead, I used Export > Using QT Conversion. I accepted all defaults except for Size - I chose Use Custom: 1024 and 576 (although perhaps Use Current Size would do).
The resultant QT movie displayed on my LCD computer monitor in 16:9 format (which it had not done before when doing Export > QT Movie) so that was a step forward.
I then dragged the QT movie into Toast 7.0.2 as usual (no need to change defaults) and the image in Toast was in 16:9. The resulting DVD displayed as 16:9 on my LCD computer monitor. (However - not surprisingly - it displayed as 4:3 on my older TV set, which is 15 years old, with the image cropped left and right - 16:9 hadn't been invented 15 years ago!)
I can't test it on my modern TV set because my wife is using it for hours, but am confident that it will show 16:9 if I set it to that.
Thanks fo the help.