Vauxhall alarm?

Hi, I own a P reg Vectra with the standard alarm system installed, the model of the car is a CDX if it makes any difference, yesterday after shopping the car would not unlock using the keyfob, it unlocked the car to go shopping and locked when we parked up, but about 1hour later, nothing. I changed the battery incase that was dead, as no lights on the fob flashed when pressing the buttons, they flash with the new battery but still will not lock or unlock the car. the only way to get in is open the door with the key but the alarm sounds untill you get the engine started, very strange stares from other people in the car park and annoying the neighbours, any help appriciated as i would like to work on it this weekend.
Thanks

Chris
May 2009