Immobiliser problem?

Hi, my disco 2001 v8 would not start the other day, I found that the passenger door had been left open all night and the disco had been left unlocked. I shut the door locked it using the fob and then unlocked it and it then started.
Today i went to start it, again it had not been locked all night, so locked and unlocked but its cranks over fine, and it has a new battery, tried both keys cant get it to start, have checked its in park or neutral.
Any ideas would be helpful, I do not have the eka code as yet, thanks

Could be any number of things, not necessarily caused by the immobiliser. See if it's getting spark and fuel and work back from there.