When Rover sold the car to the first buyer they would usually include a blade only key for emergencies. This would allow you to get into the car if you locked them inside or lost them. It would also be possible to put this key in the ignition but the car wouldn't start.
Add to that the number of members/owners that have changed the key case and forgot to swap the immobiliser over. They have only found this out when they, like you, put a key without the immobiliser in the ign and tried to start the car.
Whatever your problem is I cannot see how your trying a key without the immobiliser chip in the ignition has anything to do with it. If so there would be many more threads like this
Best guess is:
low charge on battery (enough charge to start the car once but not enough running time to recover charge before trying to start again)
loose or corroded light duty wire to starter
worn starter solenoid contacts