how can I recode my car key?

I have a vauxhall astra car and my key doesn't work properly. I bought a new battery but the key does does not always operate the central locking. I have tried inserting the key in the ignition and pressing lock and unlock and this does seem to work sometimes but not all the time, I keep on having to do it. Can you help please.

Mrs. I. Willard
November 2011