how can i fix my vauxhall astra key fob?

bought the car second hand. Car fine. Fob worked properly for two weeks then started to break down. Changed battery but it still didn't work properly. As it stands now, Alarm wont set, I'm using the blade only to lock unlock and start. LED on dash is flashing all of the time. Amazingly, alarm will sound if bonnet is lifted. Siren blasts and lights flash but stop after 30 seconds and engine starts okay. HELP