My 1999 Rover 400 diesel has an ignition problem?

My car had stood unused for a fortnight. When i try to start the car a clicking noise is coming from under the bonnet. I thought perhaps it was a flat battery, and did manage to jump start the car using leads, however, i did had to rev the other car.
Charged the battery overnight (battery is only a year old). Car still wont start and just continues to make the clicking noise. Thought about the immobiliser, so have locked and unlocked. Any ideas?