I had this problem with my Brav 1.6, too.
In the end, I had to pull off the plastic grille on the front and push the release with a big screwdriver.
It took a while, a lot of tries and a few skinned knuckles.
Once I had it open I cleaned the latch mechanism and then covered it in petroleum jelly (AKA Vaseline) to grease it and keep the water off.
Now it sticks only occasionally, and when I can't get it to open, I drive to a nearby road that's full of pot-holes and while I'm driving over it, pull the release handle. The bumps usually jog the catch open.
The secondary lever catch stops the bonnet from flying right open.
I've found that if I open the bonnet at least once a week, it never sticks.
Phil Rogers
October 2009