Hello. Did you find out how to fix this? I have exactly the same problem. It was temperamental for ages but finally just conked out.
I had to open the boot inside to repair the rear wiper motor recently and noticed how the mechanism works to undo the boot. I was wondering if it is the lock at fault or the server arm which pushes the rods when you use central locking (when you hear the 'clunk')
Now I have to manually turn a quarter turn (and you can hear the server arm inside move) to open it. Sometimes it locks, sometimes it doesn't which makes the fault harder to pinpoint. (Mine is a 1999 Astra Hatch)
Anyway, hope you got yours fixed or someone can help both of us. The parts for these things are so expensive.
Martyn
November 2008