Thanks guys - sometimes you just need a shove in the right direction.
I wanted to avoid manually setting permissions in a hidden folder, but I can set the permissions in the setup packager I use (which is MSs recommended solution).
Should be fine.
Cheers
Dan
PS. I'm still surprised that I couldn't manually replace the dsn file for another pointing to a new location. The dsn file is copied to the program files folder on install and Opus finds it in its default location. I would have thought that all the database directing was taken care of by the dsn, but presumably Opus needs the DB location as well for some reason. I don't think I'll worry about this now
PPS. Also wondering why the ProgramData folder doesn't have write permissions by default. I thought this was the point of it.