OK, so I harnessed all my sysadmin skills and created a file 'temp.lua', copied it to:
C:\Users\<insert_username_here>\AppData\Local\Temp\DCS
...edited the security options so that 'everyone' had 'full control' and now the missions load.
However, the temp.lua vanishes after each mission, so you have to keep copying it back.
At least 1.5 is now working!! :)