Thanks Ray and TechFan. Java is installed and Javaw.exe actually appears in at least three locations (System32, Arduino and Processing). At a loss …
Update: all fixed! To summarize, its to do with where Java is located. My version was located in Windows/System32 (installed in 2012). My system is 64-bit Windows. I re-installed Java to the latest version – it installed Java in Windows/SysWOW64. osFirwareUpdater then worked. The following site provided the clue (but see the comment some way down the list by Mark B (no relation) as the best explanation) http://www.pdfsam.org/miscellaneous/cannot-find-javawexe/84/.