Further Update.
Tried the following procedure:
1. Deleted ALL GPX files from the device
2. Copied over the GPX file with the modified location field to the device, (used file copy NOT 'Send to MY GPs')
3. Powered up the device and selected the Geocaching option
4. Get the message 'No Geocaches found' Funny...
5. Transferred a non-modified GXP file to the device and powered up the device again..
6. I now see the single, (un-modified) cache OK (The modified one is still missing)
7. Tried again to copy the 'modified' file to the device but this time with the 'Send to MY GPs' option
8. This time I do see both caches and 'What do you know', the modified one shows the correct modified values...
9. Re-loaded the PQ that contains ALL the caches in my local area. (including all my modified caches)
10. Rechecked the values of the modified cache as loaded in step 7... STILL OK
11. Checked other modified caches that were part of the PQ. They still show the UN-modified values.
Conclusion:
a. Modified GPX files do not get loaded correctly as a stand alone file (Maybe a parsing error of the XML file), but seem to work OK if 'sent' to the empty device.
b. I.e. if the devices is cleared of ALL GPX file then, the modified files are loaded using the 'Send to MY GPs' all is OK
Even after loading the PQ GPX file.
Seems to be a problem with how the files are parsed when loading..
But hey what do I know at least I have a work around for the time being.