From memory this is a cable release system. The release handle is on the wall of the rear wheel arch behind the passenger seat.
One of two options spring to mind Get someone to push down the o/s while you release the handle several times and see if you can free the release catch. Once open lightly oil the release .
The other point and I'm not certain if I am right but I have the feeling that the handle worked in two directions down opens one side - up open the other side. I guess if you have an owners handbook you have checked how it works.
Maurice
May 2009