Override immobiliser - alarm not responding

[Scary1]

Any help, please. I have a Toad C-5 fitted, on three occasions it has failed to respond to the key fob, the doors would not unlock and it would not dissarm. I have managed to gain entry which sets off the alarm, I then put the key into the ignition, there is no starter, or ignition, the pumps do not prime and siren is still sounding. I have then had to disconnect the battery,this then kills the siren (the back up battery in the siren must be shot otherwise the siren would continue?) I then wait a while and reconnect the battery, the alarm then resonds and I can disarm the system and start the car. I am really worried that this will not work one day. New batteries are fitted to the fob.

1. Would you think that the alarm is at fault as it will not respond to fob?
2. Would an immobiliser issue prevent the doors from unlocking?
3. Can I overide the immobiser?
4. Any ideas?

Thanks