how can I fix the central locking mechanism on my 2000 Vauxhall Vectra 1.8?

I had the same problem with a Vauxhall.
Try inserting the key in the ignition, turn the key but DO NOT start the engine. Press the lock button on the key within 30 seconds. THe locks will operate. press the unlock button and the locks should unlock. your keys EPROM should now be reprogrammed to your cars immobiliser.
Failing this it could be that during changing the battery a contact within the key has had a dry solder joint, usually indicated by the red LED not working.

Fat Tintin
April 2007