Have found a couple of things through Google. Question was first asked back in March 2018.
I still haven't found a fix but am able to edit in gimp, copy visible, paste into Irfanview the save to original folder
Finally, it was my anti virus program blocking gimp. I did a system restore last night. Uninstalled Avast and had to install Gimp again and Avast again today. When I just tried to export something from Gimp, Avast just gave a pop-up to say "we have just blocked an app on your computer - do you want to allow it?" I clicked on allow and now it seems to be ok . Has nothing to do with the folders being read only as that seems to be a default on windows computers now - has been for a while