I have found this work around.
Firstly, I installed Dolphin from the repositories via Synaptic.
Then, I logged in as Root, and, using Dolphin, went to the files that I wished to change the Write ability for the user. The window for permissions in Dolphin is much more user friendly. I changed "Others" to be able to "View and modify content". I checked "Apply to all subfolders and their contents".
I logged out and then back in again as user.
When I then opened such an altered file with Libre Office, I found that the "Read Only" label was missing at the top of the page. I could then alter the file and save it.
Until PCmanFM gets altered back along the lines mentioned by djohnston, I will use Dolphin as a workaround.