I have a 05 Vauxhall Corsa
would a low car battery ( not the fob ) be able to start the car but not initiate the central locking when the key fob is pressed ?
Very unlikely. But a dying battery in the keyfob would stop it working the central locking. I assume you have replaced the keyfob battery and need to resynchronize it. If so see here:
http://www.keyfobrepair.co.uk/Guides.htm
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cars.