Key fob not working on astra bertone 2002, lights up but doesn't lock!?

When we purchased the car the fob had no battery holder. We have carefully replaced this and all looks well. The light on the fob lights up and we have tried the trick of putting the key in the ignition and holding the buttons.... nothing! It seems to have worked for so many people but not me!
Any help would be greatly appreciated.
Thanks in anticipation.