car alarm tepramental?

HI, I have a discovery 1 with an alarm fitted that every morning is very difficult to de-activate.
it has no fob but works off the key. I insurt key into the door, the indicators flash once, and the alarm de-activates.
every morning i do the same but no light, i enter the car and alarm goes off, then no ignition, infact on one occassion the central locking locked me in.
eventually after many attempts it comes back to life then is fine all day.
I've changed the battery and read to 12v so not a drop in voltage as far as i know.
any ideas?