Exporting Media to AVI format produced a file containing only Audio. This has worked OK in the past and a day of research did not find any problem with source material or program settings.
A clue to the solution was when selecting Export Media from the File drop down menu a message box appeared about there being an update to Media Encoder for this version of Premiere Pro. (Checking for updates from the Help drop down gives the message that there are no updates for Media Encoder.)
So I looked for anomalies between Premiere Pro and Media Encoder. When exporting media the Export Settings dialog box had Microsoft AVI as the Format and PAL DV as the Preset, both video and audio are selected. When the job appeared in Media Encoder and the Export Settings are examined via the Edit drop down menu, Export Video is not selected and is greyed out!
This led me to believe that the problem was in Media Encoder.
Even though Media Encoder was the latest version 4.2.0.006 I downloaded and installed it again (without uninstalling first).
Export Video now appears selected in the Media Encoder Export Settings dialog box.
My guess is that the associated preset has been damaged?