How can I open a jammed Skoda Octavia 2003 (old model) bonnet?

How can I open a jammed Skoda Octavia 2003 (old model) bonnet. The internal bonnet release lever releases the bonnet, and I can lift the bonnet slightly. When I push the external lever it doesn't release a catch to allow me to raise the bonnet completely. I can't get at the release mechanism.