Vauxhall Astra "F" 1997. Central locking - Tailgate fails to unlock?

Symptom
Tailgate locks and unlocks using direct key insert but fails to unlock using central locking system.
Investigation
The tailgate solenoid is operating to free the lock (allowing it to be depressed) but the cam on the rear of the lock remains in the unengaged position, therefore depressing the the lock does not open the tailgate.
Assumption
That there is another function to turn the cam (as does the direct key insert by turning to unlock) to engage the cam to open the tailgate which is not operating.
Question
Have you any ideas about how to remedy this problem?