Thanks bullet_trap but its not the save/export difference that's the issue here. Gimp is saving (or rather exporting) the XMP metadata but no other apps can read it back. Now I have opened up the jpeg files and discovered that Gimp saves quoted text within single quotes whereas all the other apps enclose relevant text in double quotes. The XMP standard allows for either single or double but not a mix.
I have come to the following conclusions:
* Gimp can read single or double quoted XMP text data
* Gimp saves XMP using single quotes
* Linux apps (via libraries) can only read XMP that uses double quotes.
The fault lies with the linux libraries, however, I suspect there may be regional/character set issue here. More investigation coming.........