Warming the joint and then applying steady pressure (wedges) can help.
When you re-glue the surfaces should be free of old glue to allow the fresh stuff to penetrate the fibres.
I recall seeing someone take a joint apart using a steam wallpaper stripper with the flat end removed so steam could be applied to different parts of the joint.
Thanks for all your advice. The problem was on a small box so I got my wifes hair drier on it and the bits came apart with sligh bit of persuasion. Thanks again.
Sam