It is a brand new institute provided laptop, but used a managed software centre which restricts installation and downloads of other sources. After getting an admin account set-up I managed to download a fresh copy of fiji (fiji-macosx.dmg) and sort everything out.
There are still some minor issues, such as the console which shows
[WARN] Unkown IlluminationType value 'Fluorescence' will be stored as "Other"
and the files are all separated, not tiled and merged like I saved them originally. I can sort that out though. At least I can open the files now.