Ok, managed to resolve it myself. Here's the solution for snocked, twan and anyone else who might be interested.
Given that it would work with root but not <user> I figured it had to be a permissions problem or some other security thing.
Opened a shell and in /dev/ ls -l sr0 to see what the permissions where. These where fine - owned by <user> group disk with appropriate permissions.
ls -l sg0 gave owned by root group disk with appropriate permissions. This seemed ok - <user> was a member of group disk.
Stumped I turned to the net: someone has posted about changing owner to <user> so I tried that. now sg0 is owned by <user> with appropriate permissions (rw).
Bingo!! It works now.
Not sure why it wouldn't work with the group disk (it had r permission only set)?
Not sure why need to modify device sg0? The guy who posted the answer (Patrick - kudos) merely states that sgx are to do with audio cd things.
Anyway it works - I'll worry about why later on.