How can i fix my Mondeo central locking?

I have an N reg ford Mondeo.

The central locking doesn't work at all.

I can open the driver & passenger front doors with the key but to open the back doors I need to reach round to pull the lock out. (to lock the doors, i need to push the handles in before getting out of the car which is now getting a pain if the car has people in the front & back!!)

The light on the key lights up so the battery is not flat.

The remote central locking doesn't work at all either.

Any ideas how to fix the remote central locking or just the central locking with the key?

Any help would be appreciated.

Thanks