the battery on my rover 418 died and when i tried to start it with jump l?

the battery in my rover 418 died i tried to start it with jump leads but the alarm was going off and the car wouldnt start there is an immobiliser on car which was fitted at a later date but ive had this car for three weeks and am unsure of cars history . do i need to disconnect immobiliser and if so how ? thanks cath