W Reg Corsa 1.2 16V - Door Deadlocks Don't Work

[schindler445]

Having just bought the above Corsa and I thought I'd make sure it was safe - all locks working as the manual says.
Having put someone in the car, I turned the key in the drivers side door to the quater past nine position to activate the deadlocks.
It didn't work - you can still open the doors by pulling up the button.
Is this a common fault and does anyone know how to fix it?
Thanks.