Hi my name is Ross, I'm a car technician and I’ll help with your question.
Is there any chance the car battery could be flat not allowing the central locking to operate ?
You should be able to gain access manually by removing the black plastic plug from the key hole in the passenger side door and using your normal key blade.
This one does show up to 2004 but it depended on the country the car was sold in. Bare with me while i take a look at the manuals to find the bypass method for your model.
Ok, you should be able to unlock using the boot lock. Using the key in the boot lock should activate the central locking for all locks. If it doesn't due to a fault or flat battery it will allow manual entry into the boot so you can get access to the front.
Using the key in the boot lock should unlock all of the doors.
It may be because of a flat battery. According to the manual, the boot lock should unlock all of the doors, If the key fob is not working it will need to be re-programmed to the car using diagnostic equipment. If the key is unable to unlock the front doors, you will need to climb through to either charge the battery if that is the cause or plug in the diagnostic system to program the central locking then unlock using the fob. Unfortunately for models without the backup door lock there is no simple way around it.