Central Locking System?

I have a Vauxhall Astra sxi 1.6 3dr Aug 2005. The central locking system has stopped working. Can open driver door manually and can start engine. Passenger door stays locked and cannot be opened from inside by the button on the panel or handle. Petrol cap stays locked but boot(hatch) unlocks. I have tested batteries and they have 3.2volts.
How do I rectify, is their a relay I have to change?